Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 2,820
Distribution of Maturities: Amortized Cost, 1-5 Years	10,752
Distribution of Maturities: Amortized Cost, 5-10 Years	16,333
Distribution of Maturities: Amortized Cost, After 10 Years	1,008
Distribution of Maturities: Amortized Cost, Total	30,913
Distribution of Maturities: Fair Value, 1 Year or Less	2,884
Distribution of Maturities: Fair Value, 1-5 Years	10,781
Distribution of Maturities: Fair Value, 5-10 Years	16,287
Distribution of Maturities: Fair Value, After 10 Years	995
Distribution of Maturities: Fair Value, Total	$ 30,947
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.36%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.34%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.81%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.23%	[1]
Available For Sale Securities Debt Maturities, Yield	2.71%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 0
Distribution of Maturities: Amortized Cost, 1-5 Years	2,322
Distribution of Maturities: Amortized Cost, 5-10 Years	2,039
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	4,361
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	2,305
Distribution of Maturities: Fair Value, 5-10 Years	2,026
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	4,331
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	121
Distribution of Maturities: Amortized Cost, 1-5 Years	46
Distribution of Maturities: Amortized Cost, 5-10 Years	4
Distribution of Maturities: Amortized Cost, After 10 Years	86
Distribution of Maturities: Amortized Cost, Total	257
Distribution of Maturities: Fair Value, 1 Year or Less	123
Distribution of Maturities: Fair Value, 1-5 Years	47
Distribution of Maturities: Fair Value, 5-10 Years	4
Distribution of Maturities: Fair Value, After 10 Years	85
Distribution of Maturities: Fair Value, Total	259
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	49
Distribution of Maturities: Amortized Cost, 5-10 Years	149
Distribution of Maturities: Amortized Cost, After 10 Years	414
Distribution of Maturities: Amortized Cost, Total	618
Distribution of Maturities: Fair Value, 1 Year or Less	6
Distribution of Maturities: Fair Value, 1-5 Years	52
Distribution of Maturities: Fair Value, 5-10 Years	153
Distribution of Maturities: Fair Value, After 10 Years	406
Distribution of Maturities: Fair Value, Total	617
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	6
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
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	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	8
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	7
Distribution of Maturities: Amortized Cost, 1-5 Years	10
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	17
Distribution of Maturities: Fair Value, 1 Year or Less	7
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	17
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	2,686
Distribution of Maturities: Amortized Cost, 1-5 Years	7,937
Distribution of Maturities: Amortized Cost, 5-10 Years	11,781
Distribution of Maturities: Amortized Cost, After 10 Years	212
Distribution of Maturities: Amortized Cost, Total	22,616
Distribution of Maturities: Fair Value, 1 Year or Less	2,748
Distribution of Maturities: Fair Value, 1-5 Years	7,980
Distribution of Maturities: Fair Value, 5-10 Years	11,763
Distribution of Maturities: Fair Value, After 10 Years	213
Distribution of Maturities: Fair Value, Total	22,704
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	55
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	55
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	59
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	59
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	0
Distribution of Maturities: Amortized Cost, 1-5 Years	315
Distribution of Maturities: Amortized Cost, 5-10 Years	1,547
Distribution of Maturities: Amortized Cost, After 10 Years	259
Distribution of Maturities: Amortized Cost, Total	2,121
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	308
Distribution of Maturities: Fair Value, 5-10 Years	1,525
Distribution of Maturities: Fair Value, After 10 Years	253
Distribution of Maturities: Fair Value, Total	2,086
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	813
Distribution of Maturities: Amortized Cost, After 10 Years	37
Distribution of Maturities: Amortized Cost, Total	862
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	816
Distribution of Maturities: Fair Value, After 10 Years	38
Distribution of Maturities: Fair Value, Total	$ 866

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