Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,687
Distribution of Maturities: Amortized Cost, 1-5 Years	12,278
Distribution of Maturities: Amortized Cost, 5-10 Years	5,954
Distribution of Maturities: Amortized Cost, After 10 Years	5,277
Distribution of Maturities: Amortized Cost, Total	26,196
Distribution of Maturities: Fair Value, 1 Year or Less	2,835
Distribution of Maturities: Fair Value, 1-5 Years	12,593
Distribution of Maturities: Fair Value, 5-10 Years	6,011
Distribution of Maturities: Fair Value, After 10 Years	5,280
Distribution of Maturities: Fair Value, Total	$ 26,719
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	2.35%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.41%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.57%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	2.80%	[1]
Available For Sale Securities Debt Maturities, Yield	2.52%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 25
Distribution of Maturities: Amortized Cost, 1-5 Years	921
Distribution of Maturities: Amortized Cost, 5-10 Years	2,074
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	3,020
Distribution of Maturities: Fair Value, 1 Year or Less	25
Distribution of Maturities: Fair Value, 1-5 Years	931
Distribution of Maturities: Fair Value, 5-10 Years	2,109
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	3,065
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	159
Distribution of Maturities: Amortized Cost, 1-5 Years	109
Distribution of Maturities: Amortized Cost, 5-10 Years	14
Distribution of Maturities: Amortized Cost, After 10 Years	126
Distribution of Maturities: Amortized Cost, Total	408
Distribution of Maturities: Fair Value, 1 Year or Less	163
Distribution of Maturities: Fair Value, 1-5 Years	116
Distribution of Maturities: Fair Value, 5-10 Years	14
Distribution of Maturities: Fair Value, After 10 Years	127
Distribution of Maturities: Fair Value, Total	420
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	38
Distribution of Maturities: Amortized Cost, 1-5 Years	13
Distribution of Maturities: Amortized Cost, 5-10 Years	101
Distribution of Maturities: Amortized Cost, After 10 Years	15
Distribution of Maturities: Amortized Cost, Total	167
Distribution of Maturities: Fair Value, 1 Year or Less	39
Distribution of Maturities: Fair Value, 1-5 Years	13
Distribution of Maturities: Fair Value, 5-10 Years	106
Distribution of Maturities: Fair Value, After 10 Years	16
Distribution of Maturities: Fair Value, Total	174
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,462
Distribution of Maturities: Amortized Cost, 1-5 Years	11,098
Distribution of Maturities: Amortized Cost, 5-10 Years	3,756
Distribution of Maturities: Amortized Cost, After 10 Years	5,136
Distribution of Maturities: Amortized Cost, Total	22,452
Distribution of Maturities: Fair Value, 1 Year or Less	2,605
Distribution of Maturities: Fair Value, 1-5 Years	11,391
Distribution of Maturities: Fair Value, 5-10 Years	3,772
Distribution of Maturities: Fair Value, After 10 Years	5,137
Distribution of Maturities: Fair Value, Total	22,905
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	3
Distribution of Maturities: Amortized Cost, 1-5 Years	89
Distribution of Maturities: Amortized Cost, 5-10 Years	8
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	100
Distribution of Maturities: Fair Value, 1 Year or Less	3
Distribution of Maturities: Fair Value, 1-5 Years	91
Distribution of Maturities: Fair Value, 5-10 Years	8
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	102
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	12
Distribution of Maturities: Amortized Cost, 5-10 Years	1
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	13
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	13
Distribution of Maturities: Fair Value, 5-10 Years	2
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	15
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	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 presented on an FTE basis.