Asset Quality - Summary of Post-Modification Outstanding Recorded Investment TDRs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivables, Modifications, Rollforward [Roll Forward]
Balance at beginning of the period	$ 347	$ 333	$ 317	$ 280
Additions	58	20	133	134
Payments	(28)	(36)	(69)	(83)
Charge-offs	(11)	(2)	(15)	(16)
Balance at end of period	$ 366	$ 315	$ 366	$ 315