Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2016		Dec. 31, 2015
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 37,502		$ 35,961
Unrealized Gains, Available-for-sale securities		229		308
Unrealized Losses, Available-for-sale securities		(821)		(496)
Fair Value, Available-for-sale securities		36,910		35,773
Amortized Cost, Held-to-Maturity Securities		12,809		14,024
Unrealized Gains, Held-to-maturity securities		116		177
Unrealized Losses, Held-to-maturity securities		(87)		(36)
Held-to-maturity Securities, Fair Value		12,838		14,165
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		21,366		17,026
Unrealized Gains, Available-for-sale securities		102		142
Unrealized Losses, Available-for-sale securities		(559)		(270)
Fair Value, Available-for-sale securities		20,909		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		4,535		1,451
Unrealized Gains, Available-for-sale securities		4		2
Unrealized Losses, Available-for-sale securities		(122)		(12)
Fair Value, Available-for-sale securities		4,417		1,441
Amortized Cost, Held-to-Maturity Securities		2,465		2,945
Unrealized Gains, Held-to-maturity securities		11		20
Unrealized Losses, Held-to-maturity securities		(9)		(9)
Held-to-maturity Securities, Fair Value		2,467		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		2,139		159
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(36)		(5)
Fair Value, Available-for-sale securities		2,103		154
Amortized Cost, Held-to-Maturity Securities		1,591		1,755
Unrealized Gains, Held-to-maturity securities		59		73
Unrealized Losses, Held-to-maturity securities		(12)		(5)
Held-to-maturity Securities, Fair Value		1,638		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		3,709		4,136
Unrealized Gains, Available-for-sale securities		118		133
Unrealized Losses, Available-for-sale securities		(10)		(26)
Fair Value, Available-for-sale securities		3,817		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		3,102		10,645
Unrealized Gains, Available-for-sale securities		2		9
Unrealized Losses, Available-for-sale securities		(58)		(145)
Fair Value, Available-for-sale securities		3,046		10,509
Amortized Cost, Held-to-Maturity Securities		2,557		3,269
Unrealized Gains, Held-to-maturity securities		11		19
Unrealized Losses, Held-to-maturity securities		(10)		(11)
Held-to-maturity Securities, Fair Value		2,558		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		1,370		1,293
Unrealized Gains, Available-for-sale securities		2		11
Unrealized Losses, Available-for-sale securities		(15)		(4)
Fair Value, Available-for-sale securities		1,357		1,300
Amortized Cost, Held-to-Maturity Securities		6,176		6,029
Unrealized Gains, Held-to-maturity securities		34		63
Unrealized Losses, Held-to-maturity securities		(56)		(11)
Held-to-maturity Securities, Fair Value		6,154		6,081
Direct Agency Obligations [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		423
Unrealized Gains, Available-for-sale securities		1
Unrealized Losses, Available-for-sale securities		(4)
Fair Value, Available-for-sale securities		420
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		15		19
Unrealized Gains, Held-to-maturity securities		1		1
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		16		20
Commercial mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities				9
Unrealized Gains, Available-for-sale securities				0
Unrealized Losses, Available-for-sale securities				0
Fair Value, Available-for-sale securities				9
Home Equity [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		69		83
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(8)		(8)
Fair Value, Available-for-sale securities		61		75
Other [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		108		110
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(3)		(21)
Fair Value, Available-for-sale securities		105		89
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	522		548
Unrealized Gains, Available-for-sale securities	[1]	0		0
Unrealized Losses, Available-for-sale securities	[1]	(1)		(2)
Fair Value, Available-for-sale securities		521	[1]	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		(5)		(3)
Fair Value, Available-for-sale securities		154		161
Residential Mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		5		7
Unrealized Gains, Held-to-maturity securities		0		1
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		$ 5		$ 8

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