Amortized Cost and Fair Value of Investments in Debt Securities by Estimated Average Life (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	[1]	$ 19,459		$ 13,682
Distribution of Maturities: Amortized Cost, 1 Year or Less		2,035
Distribution of Maturities: Amortized Cost, 1-5 Years		15,198
Distribution of Maturities: Amortized Cost, 5-10 Years		12,348
Distribution of Maturities: Amortized Cost, After 10 Years		506
Distribution of Maturities: Amortized Cost, Total		30,087
Distribution of Maturities: Fair Value, 1 Year or Less		2,137
Distribution of Maturities: Fair Value, 1-5 Years		15,314
Distribution of Maturities: Fair Value, 5-10 Years		12,077
Distribution of Maturities: Fair Value, After 10 Years		502
Distribution of Maturities: Fair Value, Total		$ 30,030
Available For Sale Securities Debt Maturities, Yield, One Year Or Less	[2]	3.01%
Available For Sale Securities Debt Maturities, Yield, After One Through Five Years	[2]	2.40%
Available For Sale Securities Debt Maturities, Yield, After Five Through Ten Years	[2]	2.42%
Available For Sale Securities Debt Maturities, Yield, After Ten Years	[2]	3.14%
Available For Sale Securities Debt Maturities, Yield	[2]	2.46%
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	$ 383		129
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		464	[1]	999
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		9	[3]	38
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	[1]	19,923		14,681
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	392		167
US Treasury Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		0
Distribution of Maturities: Amortized Cost, 1-5 Years		2,344
Distribution of Maturities: Amortized Cost, 5-10 Years		3,142
Distribution of Maturities: Amortized Cost, After 10 Years		0
Distribution of Maturities: Amortized Cost, Total		5,486
Distribution of Maturities: Fair Value, 1 Year or Less		0
Distribution of Maturities: Fair Value, 1-5 Years		2,332
Distribution of Maturities: Fair Value, 5-10 Years		3,073
Distribution of Maturities: Fair Value, After 10 Years		0
Distribution of Maturities: Fair Value, Total		5,405
US Government Agencies Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		114
Distribution of Maturities: Amortized Cost, 1-5 Years		87
Distribution of Maturities: Amortized Cost, 5-10 Years		7
Distribution of Maturities: Amortized Cost, After 10 Years		102
Distribution of Maturities: Amortized Cost, Total		310
Distribution of Maturities: Fair Value, 1 Year or Less		114
Distribution of Maturities: Fair Value, 1-5 Years		91
Distribution of Maturities: Fair Value, 5-10 Years		7
Distribution of Maturities: Fair Value, After 10 Years		101
Distribution of Maturities: Fair Value, Total		313
US States and Political Subdivisions Debt Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		10
Distribution of Maturities: Amortized Cost, 1-5 Years		21
Distribution of Maturities: Amortized Cost, 5-10 Years		83
Distribution of Maturities: Amortized Cost, After 10 Years		165
Distribution of Maturities: Amortized Cost, Total		279
Distribution of Maturities: Fair Value, 1 Year or Less		10
Distribution of Maturities: Fair Value, 1-5 Years		22
Distribution of Maturities: Fair Value, 5-10 Years		86
Distribution of Maturities: Fair Value, After 10 Years		161
Distribution of Maturities: Fair Value, Total		279
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		1,889
Distribution of Maturities: Amortized Cost, 1-5 Years		12,667
Distribution of Maturities: Amortized Cost, 5-10 Years		8,847
Distribution of Maturities: Amortized Cost, After 10 Years		239
Distribution of Maturities: Amortized Cost, Total		23,642
Distribution of Maturities: Fair Value, 1 Year or Less		1,989
Distribution of Maturities: Fair Value, 1-5 Years		12,788
Distribution of Maturities: Fair Value, 5-10 Years		8,645
Distribution of Maturities: Fair Value, After 10 Years		240
Distribution of Maturities: Fair Value, Total		23,662
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		48
Distribution of Maturities: Amortized Cost, 5-10 Years		23
Distribution of Maturities: Amortized Cost, After 10 Years		0
Distribution of Maturities: Amortized Cost, Total		71
Distribution of Maturities: Fair Value, 1 Year or Less		0
Distribution of Maturities: Fair Value, 1-5 Years		50
Distribution of Maturities: Fair Value, 5-10 Years		24
Distribution of Maturities: Fair Value, After 10 Years		0
Distribution of Maturities: Fair Value, Total		74
Commercial Mortgage Backed Securities [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		245
Distribution of Maturities: Amortized Cost, After 10 Years		0
Distribution of Maturities: Amortized Cost, Total		257
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		240
Distribution of Maturities: Fair Value, After 10 Years		0
Distribution of Maturities: Fair Value, Total		252
Asset-backed Securities [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		7
Distribution of Maturities: Amortized Cost, 1-5 Years		0
Distribution of Maturities: Amortized Cost, 5-10 Years		1
Distribution of Maturities: Amortized Cost, After 10 Years		0
Distribution of Maturities: Amortized Cost, Total		8
Distribution of Maturities: Fair Value, 1 Year or Less		8
Distribution of Maturities: Fair Value, 1-5 Years		0
Distribution of Maturities: Fair Value, 5-10 Years		2
Distribution of Maturities: Fair Value, After 10 Years		0
Distribution of Maturities: Fair Value, Total		10
Other Debt Obligations [Member]
Distribution of Maturities: Amortized Cost, 1 Year or Less		15
Distribution of Maturities: Amortized Cost, 1-5 Years		19
Distribution of Maturities: Amortized Cost, 5-10 Years		0
Distribution of Maturities: Amortized Cost, After 10 Years		0
Distribution of Maturities: Amortized Cost, Total		34
Distribution of Maturities: Fair Value, 1 Year or Less		16
Distribution of Maturities: Fair Value, 1-5 Years		19
Distribution of Maturities: Fair Value, 5-10 Years		0
Distribution of Maturities: Fair Value, After 10 Years		0
Distribution of Maturities: Fair Value, Total		35
Temporarily Impaired Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		19,443		13,681
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	383		129
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		463		999
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		9	[3]	38
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		19,906		14,680
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	392		167
Temporarily Impaired Securities [Member] | US Treasury Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		4,380		2,169
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	86		14
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		0	[3]	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		4,380		2,169
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	86		14
Temporarily Impaired Securities [Member] | US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		96		75
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	2		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		3		34
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		0	[3]	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		99		109
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	2		1
Temporarily Impaired Securities [Member] | US States and Political Subdivisions Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		149
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	[3]	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		149
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	5
Temporarily Impaired Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		14,622		11,434
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	285		114
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		451		958
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		8	[3]	36
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		15,073		12,392
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	293		150
Temporarily Impaired Securities [Member] | Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		184
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	[3]	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		184
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	5
Temporarily Impaired Securities [Member] | Asset-backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		5		7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		0	[3]	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		5		7
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	0		$ 1
Temporarily Impaired Securities [Member] | Other Debt Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		12
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[3]	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	[3]	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		12
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	[3]	$ 0

[1] OTTI securities AFS are impaired securities for which OTTI credit losses have been previously recognized in earnings.

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

[3] Unrealized losses less than $0.5 million are presented as zero within the table.