Certain Transfers of Financial Assets Certain Transfers of Financial Assets - Asset Quality of Indirect Automobile Loans (Details) - Indirect automobile loan servicing rights $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Servicing Assets at Fair Value [Line Items]
Unpaid Principal Balance	$ 2,847,665
Delinquent 30 to 80	8,485
Delinquent 90 Plus	8,135
Net Charge-offs for the Year Ended December 31, 2017	7,553
Loan Servicing Portfolio
Servicing Assets at Fair Value [Line Items]
Unpaid Principal Balance	1,056,509
Delinquent 30 to 80	3,850
Delinquent 90 Plus	3,916
Net Charge-offs for the Year Ended December 31, 2017	2,874
Held-for-sale
Servicing Assets at Fair Value [Line Items]
Unpaid Principal Balance	75,000
Delinquent 30 to 80	0
Delinquent 90 Plus	0
Net Charge-offs for the Year Ended December 31, 2017	0
Held-for-investment
Servicing Assets at Fair Value [Line Items]
Unpaid Principal Balance	1,716,156
Delinquent 30 to 80	4,635
Delinquent 90 Plus	4,219
Net Charge-offs for the Year Ended December 31, 2017	$ 4,679