Securities - Details of Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
SECURITIES AVAILABLE FOR SALE
Amortized Cost	$ 19,125	$ 18,546
Gross Unrealized Gains	9	16
Gross Unrealized Losses	793	423
Fair Value	18,341	18,139
HELD-TO-MATURITY SECURITIES
Amortized Cost	11,869	11,830
Gross Unrealized Gains	0	7
Gross Unrealized Losses	608	272
Fair Value	11,261	11,565
US Treasury and Government [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	150	159
Gross Unrealized Gains	0	0
Gross Unrealized Losses	5	2
Fair Value	145	157
US States and Political Subdivisions Debt Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	7	9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	7	9
Collateralized Mortgage Obligations [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	14,804	14,985
Gross Unrealized Gains	4	10
Gross Unrealized Losses	616	335
Fair Value	14,192	14,660
HELD-TO-MATURITY SECURITIES
Amortized Cost	7,330	8,055
Gross Unrealized Gains	0	0
Gross Unrealized Losses	400	224
Fair Value	6,930	7,831
Residential Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	1,708	1,456
Gross Unrealized Gains	2	3
Gross Unrealized Losses	49	20
Fair Value	1,661	1,439
HELD-TO-MATURITY SECURITIES
Amortized Cost	508	574
Gross Unrealized Gains	0	1
Gross Unrealized Losses	22	4
Fair Value	486	571
Other Debt Obligations [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	17	17
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	0
Fair Value	20	20
HELD-TO-MATURITY SECURITIES
Amortized Cost	12	15
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	12	15
Other Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	2,439	1,920
Gross Unrealized Gains	0	0
Gross Unrealized Losses	123	66
Fair Value	2,316	1,854
HELD-TO-MATURITY SECURITIES
Amortized Cost	4,019	3,186
Gross Unrealized Gains	0	6
Gross Unrealized Losses	186	44
Fair Value	$ 3,833	$ 3,148

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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