Securities - Available for Sale Securities (Unrealized Loss Position) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	$ 461	$ 272
Fair value less than 12 months, total temporarily impaired securities	12,381	10,991
Gross unrealized losses less than 12 months, total temporarily impaired securities	283	125
Fair value 12 months or longer, total temporarily impaired securities	15,405	16,041
Gross unrealized losses 12 months or longer, total temporarily impaired securities	794	570
Fair value, total temporarily impaired securities	27,786	27,032
Gross unrealized losses, total temporarily impaired securities	1,077	695
U.S. Treasury, Agencies and Corporations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	40	41
Gross unrealized losses, less than 12 months	1	0
Fair value, 12 months or longer	106	116
Gross unrealized losses, 12 months or longer	4	2
Fair Value	146	157
Gross unrealized losses, Total	5	2
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	5,974	6,153
Gross unrealized losses, less than 12 months	125	74
Fair value, 12 months or longer	7,025	7,270
Gross unrealized losses, 12 months or longer	353	261
Fair Value	12,999	13,423
Gross unrealized losses, Total	478	335
Held-to-maturity Securities, Fair Value less than 12 months	2,137	2,201
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	56	27
Held-to-maturity Securities, Fair Value 12 months or longer	5,319	5,599
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	275	197
Held-to-maturity Securities, Fair Value	7,456	7,800
Held-to-maturity Securities, Gross unrealized losses	331	224
Agency Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	16	4
Agency Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	194	205
Gross unrealized losses, less than 12 months	7	4
Fair value, 12 months or longer	1,611	1,649
Gross unrealized losses, 12 months or longer	89	62
Fair Value	1,805	1,854
Gross unrealized losses, Total		66
Held-to-maturity Securities, Fair Value less than 12 months	3,071	1,470
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	71	12
Held-to-maturity Securities, Fair Value 12 months or longer	484	495
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	43	32
Held-to-maturity Securities, Fair Value	3,555	1,965
Held-to-maturity Securities, Gross unrealized losses	114	44
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Fair Value less than 12 months	2	3
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	0	0
Held-to-maturity Securities, Fair Value 12 months or longer	0	4
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	0	0
Held-to-maturity Securities, Fair Value	2	7
Held-to-maturity Securities, Gross unrealized losses	0	0
Gross unrealized losses, available for sale securities (less than)	1
Gross unrealized losses, held-to-maturity securities (less than)	1
Other Mortgage-Backed Securities [Member] | Agency Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	618	666
Gross unrealized losses, less than 12 months	14	7
Fair value, 12 months or longer	667	702
Gross unrealized losses, 12 months or longer	23	13
Fair Value	1,285	1,368
Gross unrealized losses, Total	37	20
Held-to-maturity Securities, Fair Value less than 12 months	345	252
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	9	1
Held-to-maturity Securities, Fair Value 12 months or longer	193	206
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	7	3
Held-to-maturity Securities, Fair Value	538	458
Held-to-maturity Securities, Gross unrealized losses	16	4
Other Mortgage-Backed Securities [Member] | Agency Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses, Total	96
Held-to-maturity Securities, Gross unrealized losses	$ 114	$ 44