Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 31,923	$ 37,502
Unrealized Gains, Available-for-sale securities		275	229
Unrealized Losses, Available-for-sale securities		(505)	(821)
Fair Value, Available-for-sale securities		31,693	36,910
Amortized Cost, Held-to-Maturity Securities		14,361	12,809
Unrealized Gains, Held-to-maturity securities		109	116
Unrealized Losses, Held-to-maturity securities		(69)	(87)
Held-to-maturity Securities, Fair Value		14,401	12,838
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		16,069	21,366
Unrealized Gains, Available-for-sale securities		147	102
Unrealized Losses, Available-for-sale securities		(331)	(559)
Fair Value, Available-for-sale securities		15,885	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		3,384	4,535
Unrealized Gains, Available-for-sale securities		5	4
Unrealized Losses, Available-for-sale securities		(64)	(122)
Fair Value, Available-for-sale securities		3,325	4,417
Amortized Cost, Held-to-Maturity Securities		2,186	2,465
Unrealized Gains, Held-to-maturity securities		17	11
Unrealized Losses, Held-to-maturity securities		(4)	(9)
Held-to-maturity Securities, Fair Value		2,199	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		1,351	1,591
Unrealized Gains, Held-to-maturity securities		49	59
Unrealized Losses, Held-to-maturity securities		(10)	(12)
Held-to-maturity Securities, Fair Value		1,390	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		3,356	3,709
Unrealized Gains, Available-for-sale securities		99	118
Unrealized Losses, Available-for-sale securities		(1)	(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		5,430	3,102
Unrealized Gains, Available-for-sale securities		9	2
Unrealized Losses, Available-for-sale securities		(64)	(58)
Fair Value, Available-for-sale securities		5,375	3,046
Amortized Cost, Held-to-Maturity Securities		2,603	2,557
Unrealized Gains, Held-to-maturity securities		11	11
Unrealized Losses, Held-to-maturity securities		(6)	(10)
Held-to-maturity Securities, Fair Value		2,608	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		999	1,370
Unrealized Gains, Available-for-sale securities		2	2
Unrealized Losses, Available-for-sale securities		(12)	(15)
Fair Value, Available-for-sale securities		989	1,357
Amortized Cost, Held-to-Maturity Securities		8,203	6,176
Unrealized Gains, Held-to-maturity securities		31	34
Unrealized Losses, Held-to-maturity securities		(49)	(56)
Held-to-maturity Securities, Fair Value		8,185	6,154
Direct Agency Obligations [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		423	423
Unrealized Gains, Available-for-sale securities		11	1
Unrealized Losses, Available-for-sale securities		0	(4)
Fair Value, Available-for-sale securities		434	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		58	69
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(3)	(8)
Fair Value, Available-for-sale securities		55	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		2	0
Unrealized Losses, Available-for-sale securities		0	(3)
Fair Value, Available-for-sale securities		509	105
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	890	522
Unrealized Gains, Available-for-sale securities	[1]	0	0
Unrealized Losses, Available-for-sale securities	[1]	(1)	(1)
Fair Value, Available-for-sale securities	[1]	889	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		4	5
Unrealized Gains, Held-to-maturity securities		0	0
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		$ 4	$ 5

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