Income Taxes - Effective tax rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income tax reconciliation
Tax expense at the U.S. Federal statutory income tax rate	$ 102	$ 152	$ 192
State and local taxes, net of Federal benefit	21	30	31
Adjustment of State tax rate used to value deferred taxes	(8)	(16)	(6)
Non-deductible FDIC assessment fees	17	11	13
Other non-deductible / non-taxable items	(3)	11	3
Items affecting prior periods	(3)	11	0
Low income housing and other tax credit investments	(30)	(22)	(15)
Other	(1)	0	6
Provision for income taxes	$ 95	$ 177	$ 224
Effective income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate (in percent)	21.00%	21.00%	21.00%
State and local taxes, net of federal benefit (in percent)	4.30%	4.10%	3.40%
Adjustment of State tax rate used to value deferred taxes (in percent)	(1.60%)	(2.20%)	(0.70%)
Non-deductible FDIC assessment fees (in percent)	3.50%	1.50%	1.40%
Other non-deductible / non-taxable items (in percent)	(0.60%)	1.50%	0.30%
Items affecting prior periods (in percent)	(0.60%)	1.50%	0.00%
Low income housing and other tax credit investments (in percent)	(6.20%)	(3.00%)	(1.70%)
Other (in percent)	(0.30%)	0.00%	0.90%
Effective tax rate (in percent)	19.50%	24.40%	24.60%