Quarterly report pursuant to Section 13 or 15(d)

Summary of Marketable Securities (Detail)

v3.5.0.2
Summary of Marketable Securities (Detail) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Schedule of Marketable Securities [Line Items]    
Trading Securities, Short-term Investments, Amortized Cost $ 1,589 $ 1,425
Trading Securities, Gross Unrealized Gain 0 9
Trading Securities, Gross Unrealized Loss (150) (5)
Trading Securities 1,439 1,429
Available-for-sale Securities, Amortized Cost Basis 615 569
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 5 1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 (3)
Available-for-sale Securities, Current 620 567
Total Amortized Cost 2,204 1,994
Marketable Securities, Gross Unrealized Gain 5 10
Marketable Securities, Gross Unrealized Loss (150) (8)
Total Estimated Fair Value 2,059 1,996
U.S. government and agency debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 321 341
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 2 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 (1)
Available-for-sale Securities, Current 323 340
Mortgage and asset-backed debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 86 74
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 1 1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 (1)
Available-for-sale Securities, Current 87 74
Corporate debt securities    
Schedule of Marketable Securities [Line Items]    
Trading Securities, Short-term Investments, Amortized Cost 1,156 715
Trading Securities, Gross Unrealized Gain 0 0
Trading Securities, Gross Unrealized Loss 0 0
Trading Securities 1,156 715
Available-for-sale Securities, Amortized Cost Basis 203 147
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 2 0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0 (1)
Available-for-sale Securities, Current 205 146
U.S. state and local municipal debt securities    
Schedule of Marketable Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis   2
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   2
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]    
Trading Securities, Short-term Investments, Amortized Cost   363
Trading Securities, Gross Unrealized Gain   0
Trading Securities, Gross Unrealized Loss   0
Trading Securities   363
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 433 347
Trading Securities, Gross Unrealized Gain 0 9
Trading Securities, Gross Unrealized Loss (150) (5)
Trading Securities $ 283 $ 351