Securities - Available for Sale Securities (Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Held-to-maturity securities:
Temporarily impaired securities, fair value, less than 12 months	$ 8,147	$ 10,991
Temporarily impaired securities, gross unrealized losses, less than 12 months	207	125
Temporarily impaired securities, fair value, 12 months or longer	20,247	16,041
Temporarily impaired securities, gross unrealized losses, 12 months or longer	1,194	570
Temporarily impaired securities, fair value	28,394	27,032
Temporarily impaired securities, gross unrealized losses	1,401	695
US Treasury and Government [Member]
Securities available for sale:
Fair value, less than 12 months	0	41
Gross unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	145	116
Gross unrealized losses, 12 months or longer	5	2
Fair value	145	157
Gross unrealized losses	5	2
Collateralized Mortgage Obligations [Member]
Securities available for sale:
Fair value, less than 12 months	3,121	6,153
Gross unrealized losses, less than 12 months	54	74
Fair value, 12 months or longer	10,062	7,270
Gross unrealized losses, 12 months or longer	562	261
Fair value	13,183	13,423
Gross unrealized losses	616	335
Held-to-maturity securities:
Fair value, less than 12 months	1,275	2,201
Gross unrealized losses, less than 12 months	44	27
Fair value, 12 months or longer	5,655	5,599
Gross unrealized losses, 12 months or longer	356	197
Fair value	6,930	7,800
Gross unrealized losses (less than $1 million for other securities)	400	224
Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Fair value, less than 12 months	558	205
Gross unrealized losses, less than 12 months	2	4
Fair value, 12 months or longer	1,758	1,649
Gross unrealized losses, 12 months or longer	121	62
Fair value	2,316	1,854
Gross unrealized losses		66
Held-to-maturity securities:
Fair value, less than 12 months	2,629	1,470
Gross unrealized losses, less than 12 months	98	12
Fair value, 12 months or longer	1,205	495
Gross unrealized losses, 12 months or longer	88	32
Fair value	3,834	1,965
Gross unrealized losses (less than $1 million for other securities)	186	44
Other Debt Obligations [Member]
Held-to-maturity securities:
Fair value, less than 12 months	2	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	2	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	409	666
Gross unrealized losses, less than 12 months	3	7
Fair value, 12 months or longer	1,089	702
Gross unrealized losses, 12 months or longer	46	13
Fair value	1,498	1,368
Gross unrealized losses	49	20
Held-to-maturity securities:
Fair value, less than 12 months	153	252
Gross unrealized losses, less than 12 months	6	1
Fair value, 12 months or longer	333	206
Gross unrealized losses, 12 months or longer	16	3
Fair value	486	458
Gross unrealized losses (less than $1 million for other securities)	22	$ 4
Other Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Gross unrealized losses	$ 123

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