INCOME TAXES - Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amount
U.S. Federal income tax expense	$ 459	$ 669	$ 537
Increase (decrease) resulting from:
Federal rate change	(34)	(331)	0
State and local income taxes (net of federal benefit)	89	46	38
Bank-owned life insurance	(12)	(19)	(19)
Tax-exempt interest	(15)	(21)	(19)
Tax advantaged investments (including related credits)	(44)	(51)	(31)
Other tax credits	(8)	(3)	(14)
Adjustments for uncertain tax positions	1	(23)	0
Non-deductible FDIC premiums	21	0	0
Other	5	(7)	(3)
Income tax expense	$ 462	$ 260	$ 489
Rate
U.S. Federal income tax rate	21.00%	35.00%	35.00%
Increase (decrease) resulting from:
Federal rate change	(1.60%)	(17.30%)	0.00%
State and local income taxes (net of federal benefit)	4.10%	2.40%	2.50%
Bank-owned life insurance	(0.50%)	(1.00%)	(1.20%)
Tax-exempt interest	(0.70%)	(1.10%)	(1.30%)
Tax advantaged investments (including related credits)	(2.00%)	(2.70%)	(2.00%)
Other tax credits	(0.40%)	(0.10%)	(0.90%)
Adjustments for uncertain tax positions	0.10%	(1.20%)	0.00%
Non-deductible FDIC premiums	1.00%	0.00%	0.00%
Other	0.20%	(0.40%)	(0.20%)
Total income tax expense and tax rate	21.20%	13.60%	31.90%