Income Taxes - Schedule of Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax rate	21.00%	21.00%	21.00%
Tax computed at statutory rate	$ 196	$ 123	$ 151
Increase (decrease) resulting from:
State income taxes, net of federal income tax benefit	9	15	25
Bank-owned life insurance	(6)	(5)	(4)
401(k) – employee stock ownership plan	(1)	(1)	(1)
Tax-exempt interest	(8)	(6)	(7)
Non-deductible expenses	13	11	8
LIHTC credits and benefits, net of amortization	(9)	(4)	(7)
Other tax credits	(5)	0	(3)
Other changes in unrecognized tax benefits	(9)	4	6
Purchase accounting gain	(112)	0	0
Effect of TCJA	0	0	(7)
Other	8	(3)	(4)
Total	$ 76	$ 134	$ 157