Securities - Available for Sale Securities (Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Held-to-maturity securities:
Temporarily impaired securities, fair value, less than 12 months	$ 11,738	$ 10,991
Temporarily impaired securities, gross unrealized losses, less than 12 months	340	125
Temporarily impaired securities, fair value, 12 months or longer	15,620	16,041
Temporarily impaired securities, gross unrealized losses, 12 months or longer	903	570
Temporarily impaired securities, fair value	27,358	27,032
Temporarily impaired securities, gross unrealized losses	1,243	695
US Treasury and Government [Member]
Securities available for sale:
Fair value, less than 12 months	39	41
Gross unrealized losses, less than 12 months	1	0
Fair value, 12 months or longer	107	116
Gross unrealized losses, 12 months or longer	3	2
Fair value	146	157
Gross unrealized losses	4	2
Collateralized Mortgage Obligations [Member]
Securities available for sale:
Fair value, less than 12 months	5,742	6,153
Gross unrealized losses, less than 12 months	141	74
Fair value, 12 months or longer	7,230	7,270
Gross unrealized losses, 12 months or longer	405	261
Fair value	12,972	13,423
Gross unrealized losses	546	335
Held-to-maturity securities:
Fair value, less than 12 months	2,058	2,201
Gross unrealized losses, less than 12 months	65	27
Fair value, 12 months or longer	5,156	5,599
Gross unrealized losses, 12 months or longer	307	197
Fair value	7,214	7,800
Gross unrealized losses (less than $1 million for other securities)	372	224
Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Fair value, less than 12 months	192	205
Gross unrealized losses, less than 12 months	9	4
Fair value, 12 months or longer	1,593	1,649
Gross unrealized losses, 12 months or longer	99	62
Fair value	1,785	1,854
Gross unrealized losses		66
Held-to-maturity securities:
Fair value, less than 12 months	3,030	1,470
Gross unrealized losses, less than 12 months	104	12
Fair value, 12 months or longer	477	495
Gross unrealized losses, 12 months or longer	50	32
Fair value	3,507	1,965
Gross unrealized losses (less than $1 million for other securities)	154	44
Other Debt Obligations [Member]
Held-to-maturity securities:
Fair value, less than 12 months	9	3
Gross unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	0	4
Gross unrealized losses, 12 months or longer	0	0
Fair value	9	7
Gross unrealized losses (less than $1 million for other securities)	1	1
Other Mortgage-Backed Securities [Member] \| Residential Mortgage Backed Securities [Member]
Securities available for sale:
Fair value, less than 12 months	338	666
Gross unrealized losses, less than 12 months	10	7
Fair value, 12 months or longer	873	702
Gross unrealized losses, 12 months or longer	32	13
Fair value	1,211	1,368
Gross unrealized losses	42	20
Held-to-maturity securities:
Fair value, less than 12 months	330	252
Gross unrealized losses, less than 12 months	10	1
Fair value, 12 months or longer	184	206
Gross unrealized losses, 12 months or longer	7	3
Fair value	514	458
Gross unrealized losses (less than $1 million for other securities)	17	$ 4
Other Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Gross unrealized losses	$ 108

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