Income Taxes - Effective tax rates (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate										$ 123	$ 367	$ 76
State and local taxes, net of federal benefit										40	28	10
Adjustment of Federal tax rate used to value deferred taxes										(31)	865	0
Adjustment of State tax rate used to value deferred taxes										2	(15)	4
Non-deductible FDIC assessment fees										27	0	0
Other non-deductible / non-taxable items										107	1	21
Items affecting prior periods										(1)	6	(2)
Uncertain tax positions										6	0	(4)
Low income housing and other tax credit investments										(5)	(17)	(17)
Stock based compensation										(2)	(8)	0
Other										0	1	1
Total income tax expense (benefit)		$ 34	$ 58	$ 82	$ 92	$ 907	$ 61	$ 108	$ 152	$ 266	1,228	$ 89
Out of period adjustment to deferred tax assets											$ 9
Effective income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate, percentage	35.00%									21.00%	35.00%	35.00%
State and local taxes, net of federal benefit, percentage										6.80%	2.70%	4.60%
Adjustment of Federal tax rate used to value deferred taxes, percentage										(5.30%)	82.40%	0.00%
Adjustment of State tax rate used to value deferred taxes, percentage										0.30%	(1.40%)	1.80%
Non-deductible FDIC assessment fees, percent										4.60%	0.00%	0.00%
Other non-deductible / non-taxable items, percentage										18.30%	0.10%	9.60%
Items affecting prior periods, percentage										(0.20%)	0.60%	(0.90%)
Uncertain tax positions, percentage										1.00%	0.00%	(1.80%)
Low income housing and other tax credit investments, percentage										(0.90%)	(1.60%)	(7.80%)
Stock-based compensation, percentage										(0.20%)	(0.80%)	0.00%
Other, percentage										0.00%	0.10%	0.50%
Effective tax rate, percentage										45.40%	117.10%	40.80%