Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 30,981	$ 37,502
Unrealized Gains, Available-for-sale securities		263	229
Unrealized Losses, Available-for-sale securities		(544)	(821)
Fair Value, Available-for-sale securities		30,700	36,910
Amortized Cost, Held-to-Maturity Securities		13,977	12,809
Unrealized Gains, Held-to-maturity securities		74	116
Unrealized Losses, Held-to-maturity securities		(149)	(87)
Held-to-maturity Securities, Fair Value		13,902	12,838
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		15,340	21,366
Unrealized Gains, Available-for-sale securities		153	102
Unrealized Losses, Available-for-sale securities		(329)	(559)
Fair Value, Available-for-sale securities		15,164	20,909
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		3,616	4,535
Unrealized Gains, Available-for-sale securities		0	4
Unrealized Losses, Available-for-sale securities		(77)	(122)
Fair Value, Available-for-sale securities		3,539	4,417
Amortized Cost, Held-to-Maturity Securities		2,113	2,465
Unrealized Gains, Held-to-maturity securities		10	11
Unrealized Losses, Held-to-maturity securities		(12)	(9)
Held-to-maturity Securities, Fair Value		2,111	2,467
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		648	2,139
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(24)	(36)
Fair Value, Available-for-sale securities		624	2,103
Amortized Cost, Held-to-Maturity Securities		1,281	1,591
Unrealized Gains, Held-to-maturity securities		41	59
Unrealized Losses, Held-to-maturity securities		(18)	(12)
Held-to-maturity Securities, Fair Value		1,304	1,638
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		3,369	3,709
Unrealized Gains, Available-for-sale securities		85	118
Unrealized Losses, Available-for-sale securities		0	(10)
Fair Value, Available-for-sale securities		3,454	3,817
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		5,152	3,102
Unrealized Gains, Available-for-sale securities		1	2
Unrealized Losses, Available-for-sale securities		(87)	(58)
Fair Value, Available-for-sale securities		5,066	3,046
Amortized Cost, Held-to-Maturity Securities		2,485	2,557
Unrealized Gains, Held-to-maturity securities		4	11
Unrealized Losses, Held-to-maturity securities		(13)	(10)
Held-to-maturity Securities, Fair Value		2,476	2,558
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		792	1,370
Unrealized Gains, Available-for-sale securities		1	2
Unrealized Losses, Available-for-sale securities		(18)	(15)
Fair Value, Available-for-sale securities		775	1,357
Amortized Cost, Held-to-Maturity Securities		8,083	6,176
Unrealized Gains, Held-to-maturity securities		18	34
Unrealized Losses, Held-to-maturity securities		(106)	(56)
Held-to-maturity Securities, Fair Value		7,995	6,154
Direct Agency Obligations [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		419	423
Unrealized Gains, Available-for-sale securities		19	1
Unrealized Losses, Available-for-sale securities		0	(4)
Fair Value, Available-for-sale securities		438	420
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		12	15
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		13	16
Home Equity [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		54	69
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(3)	(8)
Fair Value, Available-for-sale securities		51	61
Other [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		506	108
Unrealized Gains, Available-for-sale securities		4	0
Unrealized Losses, Available-for-sale securities		0	(3)
Fair Value, Available-for-sale securities		510	105
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	902	522
Unrealized Gains, Available-for-sale securities	[1]	0	0
Unrealized Losses, Available-for-sale securities	[1]	0	(1)
Fair Value, Available-for-sale securities	[1]	902	521
Equity securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		183	159
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(6)	(5)
Fair Value, Available-for-sale securities		177	154
Residential Mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		3	5
Unrealized Gains, Held-to-maturity securities		0	0
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		$ 3	$ 5

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