Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 38,338	$ 37,502
Unrealized Gains, Available-for-sale securities		262	229
Unrealized Losses, Available-for-sale securities		(689)	(821)
Fair Value, Available-for-sale securities		37,911	36,910
Amortized Cost, Held-to-Maturity Securities		13,315	12,809
Unrealized Gains, Held-to-maturity securities		108	116
Unrealized Losses, Held-to-maturity securities		(90)	(87)
Held-to-maturity Securities, Fair Value		13,333	12,838
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		22,371	21,366
Unrealized Gains, Available-for-sale securities		135	102
Unrealized Losses, Available-for-sale securities		(435)	(559)
Fair Value, Available-for-sale securities		22,071	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		5,214	4,535
Unrealized Gains, Available-for-sale securities		3	4
Unrealized Losses, Available-for-sale securities		(124)	(122)
Fair Value, Available-for-sale securities		5,093	4,417
Amortized Cost, Held-to-Maturity Securities		2,382	2,465
Unrealized Gains, Held-to-maturity securities		11	11
Unrealized Losses, Held-to-maturity securities		(10)	(9)
Held-to-maturity Securities, Fair Value		2,383	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		1,482	2,139
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(32)	(36)
Fair Value, Available-for-sale securities		1,450	2,103
Amortized Cost, Held-to-Maturity Securities		1,509	1,591
Unrealized Gains, Held-to-maturity securities		56	59
Unrealized Losses, Held-to-maturity securities		(12)	(12)
Held-to-maturity Securities, Fair Value		1,553	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		3,728	3,709
Unrealized Gains, Available-for-sale securities		115	118
Unrealized Losses, Available-for-sale securities		(4)	(10)
Fair Value, Available-for-sale securities		3,839	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		2,971	3,102
Unrealized Gains, Available-for-sale securities		2	2
Unrealized Losses, Available-for-sale securities		(64)	(58)
Fair Value, Available-for-sale securities		2,909	3,046
Amortized Cost, Held-to-Maturity Securities		2,434	2,557
Unrealized Gains, Held-to-maturity securities		7	11
Unrealized Losses, Held-to-maturity securities		(13)	(10)
Held-to-maturity Securities, Fair Value		2,428	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		1,114	1,370
Unrealized Gains, Available-for-sale securities		2	2
Unrealized Losses, Available-for-sale securities		(18)	(15)
Fair Value, Available-for-sale securities		1,098	1,357
Amortized Cost, Held-to-Maturity Securities		6,971	6,176
Unrealized Gains, Held-to-maturity securities		33	34
Unrealized Losses, Held-to-maturity securities		(55)	(56)
Held-to-maturity Securities, Fair Value		6,949	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		5	1
Unrealized Losses, Available-for-sale securities		0	(4)
Fair Value, Available-for-sale securities		424	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		14	15
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		15	16
Home Equity [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		66	69
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(7)	(8)
Fair Value, Available-for-sale securities		59	61
Other [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		507	108
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		0	(3)
Fair Value, Available-for-sale securities		507	105
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	307	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]	307	521
Equity securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		159	159
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(5)	(5)
Fair Value, Available-for-sale securities		154	154
Residential Mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		5	5
Unrealized Gains, Held-to-maturity securities		0	0
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		$ 5	$ 5

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