Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Dec. 31, 2015 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,590
Distribution of Maturities: Amortized Cost, 1-5 Years	10,645
Distribution of Maturities: Amortized Cost, 5-10 Years	9,301
Distribution of Maturities: Amortized Cost, After 10 Years	4,499
Distribution of Maturities: Amortized Cost, Total	27,035
Distribution of Maturities: Fair Value, 1 Year or Less	2,724
Distribution of Maturities: Fair Value, 1-5 Years	10,801
Distribution of Maturities: Fair Value, 5-10 Years	9,283
Distribution of Maturities: Fair Value, After 10 Years	4,484
Distribution of Maturities: Fair Value, Total	$ 27,292
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.38%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.40%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.66%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.90%	[1]
Available For Sale Securities Debt Maturities, Yield	2.57%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 0
Distribution of Maturities: Amortized Cost, 1-5 Years	1,271
Distribution of Maturities: Amortized Cost, 5-10 Years	2,189
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	3,460
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	1,265
Distribution of Maturities: Fair Value, 5-10 Years	2,184
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	3,449
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	163
Distribution of Maturities: Amortized Cost, 1-5 Years	105
Distribution of Maturities: Amortized Cost, 5-10 Years	13
Distribution of Maturities: Amortized Cost, After 10 Years	121
Distribution of Maturities: Amortized Cost, Total	402
Distribution of Maturities: Fair Value, 1 Year or Less	165
Distribution of Maturities: Fair Value, 1-5 Years	111
Distribution of Maturities: Fair Value, 5-10 Years	13
Distribution of Maturities: Fair Value, After 10 Years	122
Distribution of Maturities: Fair Value, Total	411
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	35
Distribution of Maturities: Amortized Cost, 1-5 Years	6
Distribution of Maturities: Amortized Cost, 5-10 Years	101
Distribution of Maturities: Amortized Cost, After 10 Years	14
Distribution of Maturities: Amortized Cost, Total	156
Distribution of Maturities: Fair Value, 1 Year or Less	35
Distribution of Maturities: Fair Value, 1-5 Years	7
Distribution of Maturities: Fair Value, 5-10 Years	107
Distribution of Maturities: Fair Value, After 10 Years	15
Distribution of Maturities: Fair Value, Total	164
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,383
Distribution of Maturities: Amortized Cost, 1-5 Years	9,134
Distribution of Maturities: Amortized Cost, 5-10 Years	6,997
Distribution of Maturities: Amortized Cost, After 10 Years	4,363
Distribution of Maturities: Amortized Cost, Total	22,877
Distribution of Maturities: Fair Value, 1 Year or Less	2,513
Distribution of Maturities: Fair Value, 1-5 Years	9,286
Distribution of Maturities: Fair Value, 5-10 Years	6,979
Distribution of Maturities: Fair Value, After 10 Years	4,346
Distribution of Maturities: Fair Value, Total	23,124
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	92
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	92
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	94
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	94
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	9
Distribution of Maturities: Amortized Cost, 1-5 Years	0
Distribution of Maturities: Amortized Cost, 5-10 Years	1
Distribution of Maturities: Amortized Cost, After 10 Years	1
Distribution of Maturities: Amortized Cost, Total	11
Distribution of Maturities: Fair Value, 1 Year or Less	11
Distribution of Maturities: Fair Value, 1-5 Years	0
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	1
Distribution of Maturities: Fair Value, Total	12
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	37
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	37
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	38
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 38

[1] Weighted average yields are based on amortized cost and are presented on an FTE basis.