INCOME TAXES - Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount
U.S. Federal income tax expense	$ 669	$ 537	$ 442
Increase (decrease) resulting from:
2017 Tax Legislation	(331)	0	0
State and local income taxes (net of federal benefit)	46	38	27
Bank-owned life insurance	(19)	(19)	(20)
Tax-exempt interest	(21)	(19)	(17)
Tax advantaged investments (including related credits)	(51)	(31)	(16)
Other tax credits	(3)	(14)	0
Adjustments for uncertain tax positions	(23)	0	0
Non-deductible expenses	0	0	8
Other	(7)	(3)	(1)
Income tax expense	$ 260	$ 489	$ 423
Rate
U.S. Federal income tax rate	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
2017 Tax Legislation	(17.30%)	0.00%	0.00%
State and local income taxes (net of federal benefit)	2.40%	2.50%	2.10%
Bank-owned life insurance	(1.00%)	(1.20%)	(1.60%)
Tax-exempt interest	(1.10%)	(1.30%)	(1.30%)
Tax advantaged investments (including related credits)	(2.70%)	(2.00%)	(1.20%)
Other tax credits	(0.10%)	(0.90%)	(0.00%)
Adjustments for uncertain tax positions	(1.20%)	0.00%	0.00%
Non-deductible expenses	0.00%	0.00%	0.60%
Other	(0.40%)	(0.20%)	(0.10%)
Total income tax expense and tax rate	13.60%	31.90%	33.50%