Securities - Net Securities Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Securities [Abstract]
Realized gross gains	$ 3	$ 6	$ 10	$ 21	$ 38
Realized gross losses	(1)	(4)	(1)	(17)	(11)
Total net securities gains	$ 2	$ 2	$ 9	$ 4	$ 27