Quarterly report pursuant to Section 13 or 15(d)

Summary of Marketable Securities (Detail)

v3.4.0.3
Summary of Marketable Securities (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Schedule of Marketable Securities [Line Items]      
Trading Securities, Short-term Investments, Amortized Cost $ 1,547   $ 1,425
Trading Securities, Gross Unrealized Gain 0   9
Trading Securities, Gross Unrealized Loss (156)   (5)
Trading Securities 1,391   1,429
Available-for-sale Securities, Amortized Cost Basis 986   569
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 4   1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (1)   (3)
Available-for-sale Securities, Current 989   567
Total Amortized Cost 2,533   1,994
Marketable Securities, Gross Unrealized Gain 4   10
Marketable Securities, Gross Unrealized Loss (157)   (8)
Total Estimated Fair Value 2,380   1,996
Trading Securities, Change in Unrealized Holding Gain (Loss) (156) $ 1  
U.S. government and agency debt securities      
Schedule of Marketable Securities [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 335   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 337   340
Mortgage and asset-backed debt securities      
Schedule of Marketable Securities [Line Items]      
Available-for-sale Securities, Amortized Cost Basis 72   74
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0   1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax 0   (1)
Available-for-sale Securities, Current 72   74
Corporate debt securities      
Schedule of Marketable Securities [Line Items]      
Trading Securities, Short-term Investments, Amortized Cost 910   715
Trading Securities, Gross Unrealized Gain 0   0
Trading Securities, Gross Unrealized Loss 0   0
Trading Securities 910   715
Available-for-sale Securities, Amortized Cost Basis 572   147
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 2   0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (1)   (1)
Available-for-sale Securities, Current 573   146
U.S. state and local municipal debt securities      
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
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 197   363
Trading Securities, Gross Unrealized Gain 0   0
Trading Securities, Gross Unrealized Loss 0   0
Trading Securities 197   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 440   347
Trading Securities, Gross Unrealized Gain 0   9
Trading Securities, Gross Unrealized Loss (156)   (5)
Trading Securities $ 284   $ 351