Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 1,635
Distribution of Maturities: Amortized Cost, 1-5 Years	11,273
Distribution of Maturities: Amortized Cost, 5-10 Years	16,996
Distribution of Maturities: Amortized Cost, After 10 Years	925
Distribution of Maturities: Amortized Cost, Total	30,829
Distribution of Maturities: Fair Value, 1 Year or Less	1,707
Distribution of Maturities: Fair Value, 1-5 Years	11,382
Distribution of Maturities: Fair Value, 5-10 Years	16,917
Distribution of Maturities: Fair Value, After 10 Years	921
Distribution of Maturities: Fair Value, Total	$ 30,927
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	3.51%	[1]
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	2.35%	[1]
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	2.67%	[1]
Available For Sale Securities Debt Maturities, Yield, After Ten Years	3.15%	[1]
Available For Sale Securities Debt Maturities, Yield	2.62%	[1]
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	$ 0
Distribution of Maturities: Amortized Cost, 1-5 Years	2,002
Distribution of Maturities: Amortized Cost, 5-10 Years	2,298
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	4,300
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	1,996
Distribution of Maturities: Fair Value, 5-10 Years	2,265
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	4,261
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	126
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	90
Distribution of Maturities: Amortized Cost, Total	266
Distribution of Maturities: Fair Value, 1 Year or Less	129
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	90
Distribution of Maturities: Fair Value, Total	270
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	46
Distribution of Maturities: Amortized Cost, 5-10 Years	179
Distribution of Maturities: Amortized Cost, After 10 Years	327
Distribution of Maturities: Amortized Cost, Total	558
Distribution of Maturities: Fair Value, 1 Year or Less	6
Distribution of Maturities: Fair Value, 1-5 Years	48
Distribution of Maturities: Fair Value, 5-10 Years	185
Distribution of Maturities: Fair Value, After 10 Years	324
Distribution of Maturities: Fair Value, Total	563
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	1,475
Distribution of Maturities: Amortized Cost, 1-5 Years	9,092
Distribution of Maturities: Amortized Cost, 5-10 Years	13,785
Distribution of Maturities: Amortized Cost, After 10 Years	508
Distribution of Maturities: Amortized Cost, Total	24,860
Distribution of Maturities: Fair Value, 1 Year or Less	1,544
Distribution of Maturities: Fair Value, 1-5 Years	9,199
Distribution of Maturities: Fair Value, 5-10 Years	13,730
Distribution of Maturities: Fair Value, After 10 Years	507
Distribution of Maturities: Fair Value, Total	24,980
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	59
Distribution of Maturities: Amortized Cost, 5-10 Years	0
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	59
Distribution of Maturities: Fair Value, 1 Year or Less	0
Distribution of Maturities: Fair Value, 1-5 Years	62
Distribution of Maturities: Fair Value, 5-10 Years	0
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	62
Commercial Mortgage Backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less	5
Distribution of Maturities: Amortized Cost, 1-5 Years	12
Distribution of Maturities: Amortized Cost, 5-10 Years	730
Distribution of Maturities: Amortized Cost, After 10 Years	0
Distribution of Maturities: Amortized Cost, Total	747
Distribution of Maturities: Fair Value, 1 Year or Less	5
Distribution of Maturities: Fair Value, 1-5 Years	12
Distribution of Maturities: Fair Value, 5-10 Years	733
Distribution of Maturities: Fair Value, After 10 Years	0
Distribution of Maturities: Fair Value, Total	750
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	23
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	33
Distribution of Maturities: Fair Value, 1 Year or Less	23
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	$ 33

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