Securities - Gains and Losses on Sales of AFS Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Gross gains	$ 83	$ 130	$ 86	$ 286
Gross losses	(10)	(5)	(10)	(8)
Net gains on sales of AFS debt securities	73	125	76	278
Income tax expense attributable to realized net gains on sales of AFS debt securities	$ 17	$ 48	$ 18	$ 106