Securities - Details of Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
SECURITIES AVAILABLE FOR SALE
Amortized Cost	$ 21,102	$ 19,917
Gross Unrealized Gains	126	55
Gross Unrealized Losses	374	544
Fair Value	20,854	19,428
HELD-TO-MATURITY SECURITIES
Amortized Cost	11,234	11,519
Gross Unrealized Gains	13	4
Gross Unrealized Losses	250	401
Fair Value	10,997	11,122
US Treasury and Government [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	272	150
Gross Unrealized Gains	0	0
Gross Unrealized Losses	2	3
Fair Value	270	147
US States and Political Subdivisions Debt Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	6	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	6	7
Collateralized Mortgage Obligations [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	14,277	14,315
Gross Unrealized Gains	45	20
Gross Unrealized Losses	242	373
Fair Value	14,080	13,962
HELD-TO-MATURITY SECURITIES
Amortized Cost	6,762	7,021
Gross Unrealized Gains	6	2
Gross Unrealized Losses	175	254
Fair Value	6,593	6,769
Residential Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	2,138	2,128
Gross Unrealized Gains	27	13
Gross Unrealized Losses	22	36
Fair Value	2,143	2,105
HELD-TO-MATURITY SECURITIES
Amortized Cost	475	490
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6	14
Fair Value	469	476
Other Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	17	17
Gross Unrealized Gains	8	3
Gross Unrealized Losses	0	0
Fair Value	25	20
HELD-TO-MATURITY SECURITIES
Amortized Cost	15	12
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	15	12
Other Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
SECURITIES AVAILABLE FOR SALE
Amortized Cost	4,392	3,300
Gross Unrealized Gains	46	19
Gross Unrealized Losses	108	132
Fair Value	4,330	3,187
HELD-TO-MATURITY SECURITIES
Amortized Cost	3,982	3,996
Gross Unrealized Gains	7	2
Gross Unrealized Losses	69	133
Fair Value	$ 3,920	$ 3,865

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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