Significant Accounting Policies - ASU 2016-13 (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, net of unearned income	$ 58,232		$ 31,061
Allowance for loan and lease losses	963		200	$ 180	$ 189
Other assets (deferred taxes)	20		19
Other assets	4,072		2,297
Other liabilities	1,699		990
Retained earnings	2,261		1,798
Total liabilities and equity	$ 84,209		$ 43,311
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, net of unearned income		$ 3
Allowance for loan and lease losses		(107)
Other assets (deferred taxes)		32
Other assets		(72)
Other liabilities		24
Retained earnings		(96)
Total liabilities and equity		$ (72)