Loans and Allowance for Losses and Concentrations of Credit Risk - Certain Loans Acquired Delinquencies and Net Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 47,560		$ 48,444
Net credit losses (recoveries)	16	$ 246
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	0		300
On-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	43,658		47,881
Net credit losses (recoveries)	16	246
Off-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,902		$ 563
Net credit losses (recoveries)	$ 0	$ 0