Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Sep. 30, 2016		Dec. 31, 2015
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 40,419		$ 35,961
Unrealized Gains, Available-for-sale securities		669		308
Unrealized Losses, Available-for-sale securities		(288)		(496)
Fair Value, Available-for-sale securities		40,800		35,773
Amortized Cost, Held-to-Maturity Securities		13,431		14,024
Unrealized Gains, Held-to-maturity securities		334		177
Unrealized Losses, Held-to-maturity securities		(3)		(36)
Held-to-maturity Securities, Fair Value		13,762		14,165
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		21,482		17,026
Unrealized Gains, Available-for-sale securities		385		142
Unrealized Losses, Available-for-sale securities		(220)		(270)
Fair Value, Available-for-sale securities		21,647		16,898
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	3,318		1,451
Unrealized Gains, Available-for-sale securities	[1]	40		2
Unrealized Losses, Available-for-sale securities	[1]	(1)		(12)
Fair Value, Available-for-sale securities		3,357	[1]	1,441
Amortized Cost, Held-to-Maturity Securities	[2]	2,607		2,945
Unrealized Gains, Held-to-maturity securities	[2]	77		20
Unrealized Losses, Held-to-maturity securities	[2]	0		(9)
Held-to-maturity Securities, Fair Value		2,684	[2]	2,956
Collateralized Mortgage Obligations, Issued by U.S. Government Sponsored Enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	2,244		159
Unrealized Gains, Available-for-sale securities	[1]	8		0
Unrealized Losses, Available-for-sale securities	[1]	(7)		(5)
Fair Value, Available-for-sale securities		2,245	[1]	154
Amortized Cost, Held-to-Maturity Securities	[2]	1,625		1,755
Unrealized Gains, Held-to-maturity securities	[2]	84		73
Unrealized Losses, Held-to-maturity securities	[2]	0		(5)
Held-to-maturity Securities, Fair Value		1,709	[2]	1,823
Direct Agency Obligations U S Government Sponsored Enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	4,335		4,136
Unrealized Gains, Available-for-sale securities	[1]	159		133
Unrealized Losses, Available-for-sale securities	[1]	(20)		(26)
Fair Value, Available-for-sale securities		4,474	[1]	4,243
Mortgage-backed Securities, Issued or Guaranteed by US Government Agency [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		6,385		10,645
Unrealized Gains, Available-for-sale securities		57		9
Unrealized Losses, Available-for-sale securities		(4)		(145)
Fair Value, Available-for-sale securities		6,438		10,509
Amortized Cost, Held-to-Maturity Securities		2,753		3,269
Unrealized Gains, Held-to-maturity securities		69		19
Unrealized Losses, Held-to-maturity securities		0		(11)
Held-to-maturity Securities, Fair Value		2,822		3,277
Collateralized Mortgage Obligations, Issued or Guaranteed by US Government Agency [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		1,443		1,293
Unrealized Gains, Available-for-sale securities		20		11
Unrealized Losses, Available-for-sale securities		(2)		(4)
Fair Value, Available-for-sale securities		1,461		1,300
Amortized Cost, Held-to-Maturity Securities		6,424		6,029
Unrealized Gains, Held-to-maturity securities		102		63
Unrealized Losses, Held-to-maturity securities		(3)		(11)
Held-to-maturity Securities, Fair Value		6,523		6,081
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities				340
Unrealized Gains, Available-for-sale securities				8
Unrealized Losses, Available-for-sale securities				0
Fair Value, Available-for-sale securities				348
Amortized Cost, Held-to-Maturity Securities		16		19
Unrealized Gains, Held-to-maturity securities		1		1
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		17		20
Commercial mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		4		9
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		0		0
Fair Value, Available-for-sale securities		4		9
Home Equity [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		74		83
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(9)		(8)
Fair Value, Available-for-sale securities		65		75
Other [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		517		110
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(23)		(21)
Fair Value, Available-for-sale securities		494		89
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[3]	458		548
Unrealized Gains, Available-for-sale securities	[3]	0		0
Unrealized Losses, Available-for-sale securities	[3]	(1)		(2)
Fair Value, Available-for-sale securities		457	[3]	546
Equity securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		159		161
Unrealized Gains, Available-for-sale securities		0		3
Unrealized Losses, Available-for-sale securities		(1)		(3)
Fair Value, Available-for-sale securities		158		161
Residential Mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		6		7
Unrealized Gains, Held-to-maturity securities		1		1
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		7		8
US Government sponsored enterprises [Member] | Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		3,076		1,577
Amortized Cost, Held-to-Maturity Securities		2,913		3,182
US Government sponsored enterprises [Member] | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		2,486		33
Amortized Cost, Held-to-Maturity Securities		$ 1,319		$ 1,518

[1] Includes securities at amortized cost of $3,076 million and $1,577 million issued or guaranteed by FNMA at September 30, 2016 and December 31, 2015, respectively, and $2,486 million and $33 million issued or guaranteed by FHLMC at September 30, 2016 and December 31, 2015, respectively.

[2] Includes securities at amortized cost of $2,913 million and $3,182 million issued or guaranteed by FNMA at September 30, 2016 and December 31, 2015, respectively, and $1,319 million and $1,518 million issued and guaranteed by FHLMC at September 30, 2016 and December 31, 2015, respectively.

[3] Foreign debt securities represent public sector entity, bank or corporate debt.