Loans and Allowance for Losses and Concentrations of Credit Risk - Certain Loans Acquired Delinquencies and Net Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 48,444	$ 21,038
Net credit losses (recoveries)	(1,397)	154	$ 3,853
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	300	100
On-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	47,881	19,757
Net credit losses (recoveries)	(1,397)	154	3,853
Off-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	563	1,281
Net credit losses (recoveries)	$ 0	$ 0	$ 0