Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,674
Distribution of Maturities: Amortized Cost, 1-5 Years	9,209
Distribution of Maturities: Amortized Cost, 5-10 Years	19,582
Distribution of Maturities: Amortized Cost, After 10 Years	1,162
Distribution of Maturities: Amortized Cost, Total	31,627
Distribution of Maturities: Fair Value, 1 Year or Less	1,733
Distribution of Maturities: Fair Value, 1-5 Years	9,073
Distribution of Maturities: Fair Value, 5-10 Years	19,013
Distribution of Maturities: Fair Value, After 10 Years	1,123
Distribution of Maturities: Fair Value, Total	$ 30,942
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.19%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.36%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.87%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.12%	[1]
Available For Sale Securities Debt Maturities, Yield	2.74%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 15
Distribution of Maturities: Amortized Cost, 1-5 Years	2,517
Distribution of Maturities: Amortized Cost, 5-10 Years	1,707
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	4,239
Distribution of Maturities: Fair Value, 1 Year or Less	15
Distribution of Maturities: Fair Value, 1-5 Years	2,454
Distribution of Maturities: Fair Value, 5-10 Years	1,657
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	4,126
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	117
Distribution of Maturities: Amortized Cost, 1-5 Years	40
Distribution of Maturities: Amortized Cost, 5-10 Years	4
Distribution of Maturities: Amortized Cost, After 10 Years	83
Distribution of Maturities: Amortized Cost, Total	244
Distribution of Maturities: Fair Value, 1 Year or Less	117
Distribution of Maturities: Fair Value, 1-5 Years	41
Distribution of Maturities: Fair Value, 5-10 Years	4
Distribution of Maturities: Fair Value, After 10 Years	81
Distribution of Maturities: Fair Value, Total	243
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	3
Distribution of Maturities: Amortized Cost, 1-5 Years	61
Distribution of Maturities: Amortized Cost, 5-10 Years	37
Distribution of Maturities: Amortized Cost, After 10 Years	531
Distribution of Maturities: Amortized Cost, Total	632
Distribution of Maturities: Fair Value, 1 Year or Less	4
Distribution of Maturities: Fair Value, 1-5 Years	62
Distribution of Maturities: Fair Value, 5-10 Years	38
Distribution of Maturities: Fair Value, After 10 Years	511
Distribution of Maturities: Fair Value, Total	615
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	7
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	15
Distribution of Maturities: Fair Value, 1 Year or Less	7
Distribution of Maturities: Fair Value, 1-5 Years	8
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	15
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,531
Distribution of Maturities: Amortized Cost, 1-5 Years	6,089
Distribution of Maturities: Amortized Cost, 5-10 Years	14,985
Distribution of Maturities: Amortized Cost, After 10 Years	278
Distribution of Maturities: Amortized Cost, Total	22,883
Distribution of Maturities: Fair Value, 1 Year or Less	1,589
Distribution of Maturities: Fair Value, 1-5 Years	6,031
Distribution of Maturities: Fair Value, 5-10 Years	14,566
Distribution of Maturities: Fair Value, After 10 Years	273
Distribution of Maturities: Fair Value, Total	22,459
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1
Distribution of Maturities: Amortized Cost, 1-5 Years	482
Distribution of Maturities: Amortized Cost, 5-10 Years	1,911
Distribution of Maturities: Amortized Cost, After 10 Years	270
Distribution of Maturities: Amortized Cost, Total	2,664
Distribution of Maturities: Fair Value, 1 Year or Less	1
Distribution of Maturities: Fair Value, 1-5 Years	465
Distribution of Maturities: Fair Value, 5-10 Years	1,844
Distribution of Maturities: Fair Value, After 10 Years	258
Distribution of Maturities: Fair Value, Total	2,568
Commercial Mortgage Backed Securities [Member] | 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	12
Distribution of Maturities: Amortized Cost, 5-10 Years	938
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	950
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	12
Distribution of Maturities: Fair Value, 5-10 Years	904
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 916

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