Quarterly report pursuant to Section 13 or 15(d)

Summary of Marketable Securities (Detail)

v3.7.0.1
Summary of Marketable Securities (Detail) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Schedule of Marketable Securities [Line Items]    
Trading Securities, Short-term Investments, Amortized Cost $ 440 $ 507
Trading Securities, Gross Unrealized Gain 1 10
Trading Securities, Gross Unrealized Loss 0 0
Trading Securities 441 517
Available-for-sale Securities, Amortized Cost Basis 620 576
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 2 1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (3) (3)
Available-for-sale Securities, Current 619 574
Total Amortized Cost 1,060 1,083
Marketable Securities, Gross Unrealized Gain 3 11
Marketable Securities, Gross Unrealized Loss (3) (3)
Total Estimated Fair Value 1,060 1,091
U.S. government and agency debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 289 314
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (2) (2)
Available-for-sale Securities, Current 287 312
Mortgage and asset-backed debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 91 90
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 1 1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 0
Available-for-sale Securities, Current 92 91
Corporate debt securities    
Schedule of Marketable Securities [Line Items]    
Trading Securities, Short-term Investments, Amortized Cost 353 427
Trading Securities, Gross Unrealized Gain 0 0
Trading Securities, Gross Unrealized Loss 0 0
Trading Securities 353 427
Available-for-sale Securities, Amortized Cost Basis 197 167
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 1 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (1) (1)
Available-for-sale Securities, Current 197 166
U.S. state and local municipal debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 38  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0  
Available-for-sale Securities, Current 38  
Equity Securities [Member]    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 2 2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 0
Available-for-sale Securities, Current 2 2
Foreign Government Debt Securities [Member]    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 3 3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 0
Available-for-sale Securities, Current 3 3
Carbon Credit Investments [Member]    
Schedule of Marketable Securities [Line Items]    
Trading Securities, Short-term Investments, Amortized Cost 87 80
Trading Securities, Gross Unrealized Gain 1 10
Trading Securities, Gross Unrealized Loss 0 0
Trading Securities $ 88 $ 90