Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Jun. 30, 2016 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,346
Distribution of Maturities: Amortized Cost, 1-5 Years	11,549
Distribution of Maturities: Amortized Cost, 5-10 Years	13,227
Distribution of Maturities: Amortized Cost, After 10 Years	602
Distribution of Maturities: Amortized Cost, Total	27,724
Distribution of Maturities: Fair Value, 1 Year or Less	2,458
Distribution of Maturities: Fair Value, 1-5 Years	11,938
Distribution of Maturities: Fair Value, 5-10 Years	13,628
Distribution of Maturities: Fair Value, After 10 Years	618
Distribution of Maturities: Fair Value, Total	$ 28,642
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	2.40%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.56%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.04%	[1]
Available For Sale Securities Debt Maturities, Yield	2.50%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 0
Distribution of Maturities: Amortized Cost, 1-5 Years	1,646
Distribution of Maturities: Amortized Cost, 5-10 Years	2,627
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	4,273
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	1,685
Distribution of Maturities: Fair Value, 5-10 Years	2,756
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	4,441
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	145
Distribution of Maturities: Amortized Cost, 1-5 Years	91
Distribution of Maturities: Amortized Cost, 5-10 Years	13
Distribution of Maturities: Amortized Cost, After 10 Years	112
Distribution of Maturities: Amortized Cost, Total	361
Distribution of Maturities: Fair Value, 1 Year or Less	146
Distribution of Maturities: Fair Value, 1-5 Years	98
Distribution of Maturities: Fair Value, 5-10 Years	13
Distribution of Maturities: Fair Value, After 10 Years	115
Distribution of Maturities: Fair Value, Total	372
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	26
Distribution of Maturities: Amortized Cost, 1-5 Years	16
Distribution of Maturities: Amortized Cost, 5-10 Years	89
Distribution of Maturities: Amortized Cost, After 10 Years	33
Distribution of Maturities: Amortized Cost, Total	164
Distribution of Maturities: Fair Value, 1 Year or Less	26
Distribution of Maturities: Fair Value, 1-5 Years	17
Distribution of Maturities: Fair Value, 5-10 Years	98
Distribution of Maturities: Fair Value, After 10 Years	34
Distribution of Maturities: Fair Value, Total	175
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,174
Distribution of Maturities: Amortized Cost, 1-5 Years	9,671
Distribution of Maturities: Amortized Cost, 5-10 Years	10,498
Distribution of Maturities: Amortized Cost, After 10 Years	457
Distribution of Maturities: Amortized Cost, Total	22,800
Distribution of Maturities: Fair Value, 1 Year or Less	2,285
Distribution of Maturities: Fair Value, 1-5 Years	10,009
Distribution of Maturities: Fair Value, 5-10 Years	10,761
Distribution of Maturities: Fair Value, After 10 Years	469
Distribution of Maturities: Fair Value, Total	23,524
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	82
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	82
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	83
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	83
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1
Distribution of Maturities: Amortized Cost, 1-5 Years	8
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	9
Distribution of Maturities: Fair Value, 1 Year or Less	1
Distribution of Maturities: Fair Value, 1-5 Years	10
Distribution of Maturities: Fair Value, 5-10 Years	0
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	35
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	35
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	36
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 36

[1] Weighted average yields are based on amortized cost.