Securities - Details of Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
SECURITIES AVAILABLE FOR SALE
Amortized Cost	$ 18,057	$ 18,546
Gross Unrealized Gains	10	16
Gross Unrealized Losses	700	423
Fair Value	17,367	18,139
HELD-TO-MATURITY SECURITIES
Amortized Cost	12,277	11,830
Gross Unrealized Gains	1	7
Gross Unrealized Losses	543	272
Fair Value	11,735	11,565
US Treasury and Government [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	150	159
Gross Unrealized Gains	0	0
Gross Unrealized Losses	4	2
Fair Value	146	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	14,435	14,985
Gross Unrealized Gains	5	10
Gross Unrealized Losses	546	335
Fair Value	13,894	14,660
HELD-TO-MATURITY SECURITIES
Amortized Cost	7,699	8,055
Gross Unrealized Gains	1	0
Gross Unrealized Losses	372	224
Fair Value	7,328	7,831
Residential Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	1,407	1,456
Gross Unrealized Gains	2	3
Gross Unrealized Losses	42	20
Fair Value	1,367	1,439
HELD-TO-MATURITY SECURITIES
Amortized Cost	531	574
Gross Unrealized Gains	0	1
Gross Unrealized Losses	17	4
Fair Value	514	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	15	15
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	15	15
Other Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	2,041	1,920
Gross Unrealized Gains	0	0
Gross Unrealized Losses	108	66
Fair Value	1,933	1,854
HELD-TO-MATURITY SECURITIES
Amortized Cost	4,032	3,186
Gross Unrealized Gains	0	6
Gross Unrealized Losses	154	44
Fair Value	$ 3,878	$ 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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