Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,592
Distribution of Maturities: Amortized Cost, 1-5 Years	8,086
Distribution of Maturities: Amortized Cost, 5-10 Years	20,527
Distribution of Maturities: Amortized Cost, After 10 Years	1,253
Distribution of Maturities: Amortized Cost, Total	31,458
Distribution of Maturities: Fair Value, 1 Year or Less	1,649
Distribution of Maturities: Fair Value, 1-5 Years	7,996
Distribution of Maturities: Fair Value, 5-10 Years	20,070
Distribution of Maturities: Fair Value, After 10 Years	1,219
Distribution of Maturities: Fair Value, Total	$ 30,934
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.31%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.18%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.86%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.04%	[1]
Available For Sale Securities Debt Maturities, Yield	2.72%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 0
Distribution of Maturities: Amortized Cost, 1-5 Years	2,731
Distribution of Maturities: Amortized Cost, 5-10 Years	1,706
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	4,437
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	2,673
Distribution of Maturities: Fair Value, 5-10 Years	1,667
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	4,340
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	116
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	88
Distribution of Maturities: Amortized Cost, Total	248
Distribution of Maturities: Fair Value, 1 Year or Less	118
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	86
Distribution of Maturities: Fair Value, Total	249
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	6
Distribution of Maturities: Amortized Cost, 1-5 Years	59
Distribution of Maturities: Amortized Cost, 5-10 Years	63
Distribution of Maturities: Amortized Cost, After 10 Years	516
Distribution of Maturities: Amortized Cost, Total	644
Distribution of Maturities: Fair Value, 1 Year or Less	6
Distribution of Maturities: Fair Value, 1-5 Years	61
Distribution of Maturities: Fair Value, 5-10 Years	65
Distribution of Maturities: Fair Value, After 10 Years	504
Distribution of Maturities: Fair Value, Total	636
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	0
Distribution of Maturities: Amortized Cost, 5-10 Years	5
Distribution of Maturities: Amortized Cost, After 10 Years	1
Distribution of Maturities: Amortized Cost, Total	6
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	0
Distribution of Maturities: Fair Value, 5-10 Years	6
Distribution of Maturities: Fair Value, After 10 Years	1
Distribution of Maturities: Fair Value, Total	7
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	7
Distribution of Maturities: Amortized Cost, 1-5 Years	9
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	16
Distribution of Maturities: Fair Value, 1 Year or Less	7
Distribution of Maturities: Fair Value, 1-5 Years	9
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	16
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,462
Distribution of Maturities: Amortized Cost, 1-5 Years	4,771
Distribution of Maturities: Amortized Cost, 5-10 Years	16,222
Distribution of Maturities: Amortized Cost, After 10 Years	382
Distribution of Maturities: Amortized Cost, Total	22,837
Distribution of Maturities: Fair Value, 1 Year or Less	1,517
Distribution of Maturities: Fair Value, 1-5 Years	4,747
Distribution of Maturities: Fair Value, 5-10 Years	15,877
Distribution of Maturities: Fair Value, After 10 Years	372
Distribution of Maturities: Fair Value, Total	22,513
Residential 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	49
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	4
Distribution of Maturities: Amortized Cost, Total	53
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	53
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	4
Distribution of Maturities: Fair Value, Total	57
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	414
Distribution of Maturities: Amortized Cost, 5-10 Years	1,643
Distribution of Maturities: Amortized Cost, After 10 Years	262
Distribution of Maturities: Amortized Cost, Total	2,320
Distribution of Maturities: Fair Value, 1 Year or Less	1
Distribution of Maturities: Fair Value, 1-5 Years	400
Distribution of Maturities: Fair Value, 5-10 Years	1,589
Distribution of Maturities: Fair Value, After 10 Years	252
Distribution of Maturities: Fair Value, Total	2,242
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	13
Distribution of Maturities: Amortized Cost, 5-10 Years	884
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	897
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	862
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	$ 874

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