Quarterly report pursuant to Section 13 or 15(d)

Investments Measured at Fair Value on a Recurring Basis (Detail)

v3.7.0.1
Investments Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments $ 1,088 $ 1,122
Marketable securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 1,060 1,091
Marketable securities | U.S. government and agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 287 312
Marketable securities | Mortgage and asset-backed debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 92 91
Marketable securities | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 550 593
Marketable securities | U.S. state and local municipal debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 38  
Marketable securities | Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 2 2
Marketable securities | Foreign Government Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 3 3
Marketable securities | Carbon Credit Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 88 90
Other Long-term Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 28 31
Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 394 420
Fair Value, Inputs, Level 1 | Marketable securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 375 402
Fair Value, Inputs, Level 1 | Marketable securities | U.S. government and agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 287 312
Fair Value, Inputs, Level 1 | Marketable securities | Mortgage and asset-backed debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 1 | Marketable securities | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 1 | Marketable securities | U.S. state and local municipal debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Fair Value, Inputs, Level 1 | Marketable securities | Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 1 | Marketable securities | Foreign Government Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 1 | Marketable securities | Carbon Credit Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 88 90
Fair Value, Inputs, Level 1 | Other Long-term Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 19 18
Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 685 689
Fair Value, Inputs, Level 2 | Marketable securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 685 689
Fair Value, Inputs, Level 2 | Marketable securities | U.S. government and agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 2 | Marketable securities | Mortgage and asset-backed debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 92 91
Fair Value, Inputs, Level 2 | Marketable securities | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 550 593
Fair Value, Inputs, Level 2 | Marketable securities | U.S. state and local municipal debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 38  
Fair Value, Inputs, Level 2 | Marketable securities | Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 2 2
Fair Value, Inputs, Level 2 | Marketable securities | Foreign Government Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 3 3
Fair Value, Inputs, Level 2 | Marketable securities | Carbon Credit Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 2 | Other Long-term Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 9 13
Fair Value, Inputs, Level 3 | Marketable securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | U.S. government and agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | Mortgage and asset-backed debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | U.S. state and local municipal debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Fair Value, Inputs, Level 3 | Marketable securities | Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | Foreign Government Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Marketable securities | Carbon Credit Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Inputs, Level 3 | Other Long-term Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments $ 9 $ 13