Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Mar. 31, 2016 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,425
Distribution of Maturities: Amortized Cost, 1-5 Years	10,211
Distribution of Maturities: Amortized Cost, 5-10 Years	10,551
Distribution of Maturities: Amortized Cost, After 10 Years	3,782
Distribution of Maturities: Amortized Cost, Total	26,969
Distribution of Maturities: Fair Value, 1 Year or Less	2,545
Distribution of Maturities: Fair Value, 1-5 Years	10,474
Distribution of Maturities: Fair Value, 5-10 Years	10,804
Distribution of Maturities: Fair Value, After 10 Years	3,847
Distribution of Maturities: Fair Value, Total	$ 27,670
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.50%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.35%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.69%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.91%	[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	1,371
Distribution of Maturities: Amortized Cost, 5-10 Years	2,323
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	3,694
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	1,395
Distribution of Maturities: Fair Value, 5-10 Years	2,402
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	3,797
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	142
Distribution of Maturities: Amortized Cost, 1-5 Years	106
Distribution of Maturities: Amortized Cost, 5-10 Years	13
Distribution of Maturities: Amortized Cost, After 10 Years	116
Distribution of Maturities: Amortized Cost, Total	377
Distribution of Maturities: Fair Value, 1 Year or Less	144
Distribution of Maturities: Fair Value, 1-5 Years	113
Distribution of Maturities: Fair Value, 5-10 Years	13
Distribution of Maturities: Fair Value, After 10 Years	119
Distribution of Maturities: Fair Value, Total	389
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	29
Distribution of Maturities: Amortized Cost, 1-5 Years	6
Distribution of Maturities: Amortized Cost, 5-10 Years	100
Distribution of Maturities: Amortized Cost, After 10 Years	14
Distribution of Maturities: Amortized Cost, Total	149
Distribution of Maturities: Fair Value, 1 Year or Less	30
Distribution of Maturities: Fair Value, 1-5 Years	6
Distribution of Maturities: Fair Value, 5-10 Years	108
Distribution of Maturities: Fair Value, After 10 Years	14
Distribution of Maturities: Fair Value, Total	158
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,252
Distribution of Maturities: Amortized Cost, 1-5 Years	8,598
Distribution of Maturities: Amortized Cost, 5-10 Years	8,113
Distribution of Maturities: Amortized Cost, After 10 Years	3,652
Distribution of Maturities: Amortized Cost, Total	22,615
Distribution of Maturities: Fair Value, 1 Year or Less	2,368
Distribution of Maturities: Fair Value, 1-5 Years	8,828
Distribution of Maturities: Fair Value, 5-10 Years	8,280
Distribution of Maturities: Fair Value, After 10 Years	3,714
Distribution of Maturities: Fair Value, Total	23,190
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	88
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	88
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	88
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	88
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2
Distribution of Maturities: Amortized Cost, 1-5 Years	6
Distribution of Maturities: Amortized Cost, 5-10 Years	2
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	10
Distribution of Maturities: Fair Value, 1 Year or Less	3
Distribution of Maturities: Fair Value, 1-5 Years	7
Distribution of Maturities: Fair Value, 5-10 Years	1
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	11
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	36
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	36
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	37
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 37

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