Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 30,759	$ 30,981
Unrealized Gains, Available-for-sale securities		267	263
Unrealized Losses, Available-for-sale securities		(803)	(544)
Fair Value, Available-for-sale securities		30,223	30,700
Amortized Cost, Held-to-Maturity Securities		14,274	13,977
Unrealized Gains, Held-to-maturity securities		54	74
Unrealized Losses, Held-to-maturity securities		(323)	(149)
Held-to-maturity Securities, Fair Value		14,005	13,902
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		14,847	15,340
Unrealized Gains, Available-for-sale securities		147	153
Unrealized Losses, Available-for-sale securities		(407)	(329)
Fair Value, Available-for-sale securities		14,587	15,164
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		3,779	3,616
Unrealized Gains, Available-for-sale securities		1	0
Unrealized Losses, Available-for-sale securities		(159)	(77)
Fair Value, Available-for-sale securities		3,621	3,539
Amortized Cost, Held-to-Maturity Securities		2,008	2,113
Unrealized Gains, Held-to-maturity securities		1	10
Unrealized Losses, Held-to-maturity securities		(40)	(12)
Held-to-maturity Securities, Fair Value		1,969	2,111
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		647	648
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(38)	(24)
Fair Value, Available-for-sale securities		609	624
Amortized Cost, Held-to-Maturity Securities		1,221	1,281
Unrealized Gains, Held-to-maturity securities		36	41
Unrealized Losses, Held-to-maturity securities		(28)	(18)
Held-to-maturity Securities, Fair Value		1,229	1,304
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		3,262	3,369
Unrealized Gains, Available-for-sale securities		75	85
Unrealized Losses, Available-for-sale securities		(1)	0
Fair Value, Available-for-sale securities		3,336	3,454
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		5,312	5,152
Unrealized Gains, Available-for-sale securities		1	1
Unrealized Losses, Available-for-sale securities		(167)	(87)
Fair Value, Available-for-sale securities		5,146	5,066
Amortized Cost, Held-to-Maturity Securities		2,385	2,485
Unrealized Gains, Held-to-maturity securities		3	4
Unrealized Losses, Held-to-maturity securities		(69)	(13)
Held-to-maturity Securities, Fair Value		2,319	2,476
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		775	792
Unrealized Gains, Available-for-sale securities		16	1
Unrealized Losses, Available-for-sale securities		(28)	(18)
Fair Value, Available-for-sale securities		763	775
Amortized Cost, Held-to-Maturity Securities		8,645	8,083
Unrealized Gains, Held-to-maturity securities		13	18
Unrealized Losses, Held-to-maturity securities		(186)	(106)
Held-to-maturity Securities, Fair Value		8,472	7,995
Direct Agency Obligations [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		345	419
Unrealized Gains, Available-for-sale securities		22	19
Unrealized Losses, Available-for-sale securities		0	0
Fair Value, Available-for-sale securities		367	438
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	12
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		13	13
Home Equity [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		52	54
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(3)	(3)
Fair Value, Available-for-sale securities		49	51
Other [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		303	506
Unrealized Gains, Available-for-sale securities		4	4
Unrealized Losses, Available-for-sale securities		0	0
Fair Value, Available-for-sale securities		307	510
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	1,437	902
Unrealized Gains, Available-for-sale securities	[1]	1	0
Unrealized Losses, Available-for-sale securities	[1]	0	0
Fair Value, Available-for-sale securities	[1]	1,438	902
Equity securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities			183
Unrealized Gains, Available-for-sale securities			0
Unrealized Losses, Available-for-sale securities			(6)
Fair Value, Available-for-sale securities			177
Residential Mortgages [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		3	3
Unrealized Gains, Held-to-maturity securities		0	0
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		$ 3	$ 3

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