INCOME TAXES - Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amount
U.S. Federal income tax expense	$ 473	$ 459	$ 669
Increase (decrease) resulting from:
Federal rate change	0	(34)	(331)
State and local income taxes (net of federal benefit)	73	89	46
Bank-owned life insurance	(12)	(12)	(19)
Tax-exempt interest	(15)	(15)	(21)
Tax advantaged investments (including related credits)	(50)	(44)	(51)
Other tax credits	(10)	(8)	(3)
Adjustments for uncertain tax positions	0	1	(23)
Non-deductible FDIC premiums	13	21	0
Legacy tax matters	(19)	0	0
Other	7	5	(7)
Income tax expense	$ 460	$ 462	$ 260
Rate
U.S. Federal income tax rate	21.00%	21.00%	35.00%
Increase (decrease) resulting from:
Federal rate change	0.00%	(1.60%)	(17.30%)
State and local income taxes (net of federal benefit)	3.20%	4.10%	2.40%
Bank-owned life insurance	(0.50%)	(0.50%)	(1.00%)
Tax-exempt interest	(0.70%)	(0.70%)	(1.10%)
Tax advantaged investments (including related credits)	(2.30%)	(2.00%)	(2.70%)
Other tax credits	(0.40%)	(0.40%)	(0.10%)
Adjustments for uncertain tax positions	0.00%	0.10%	(1.20%)
Non-deductible FDIC premiums	0.60%	1.00%	0.00%
Legacy tax matters	(0.80%)	0.00%	0.00%
Other	0.30%	0.20%	(0.40%)
Total income tax expense and tax rate	20.40%	21.20%	13.60%