Loans and Allowance for Loan Losses - Loan Transfers from Held to Investment to Held for Sale and Proceeds from Sales of Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Loans disclosures
Net transfer of loans held for investment to (from) loans held for sale	$ 24	$ 126	$ (67)	$ 356
Proceeds from sale	274	299	514	646
Commercial
Loans disclosures
Net transfer of loans held for investment to (from) loans held for sale	24	126	(67)	360
Proceeds from sale	274	299	514	646
Consumer
Loans disclosures
Net transfer of loans held for investment to (from) loans held for sale	0	0	0	(4)
Proceeds from sale	$ 0	$ 0	$ 0	$ 0