Asset Quality - Changes in Liability for Credit Losses on Lending Related Commitments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 60	$ 48	$ 57	$ 55
Provision (credit) for losses on lending-related commitments	(2)	0	1	(7)
Balance at end of period	$ 58	$ 48	$ 58	$ 48