Loans and Allowance for Losses and Concentrations of Credit Risk - Certain Loans Acquired Delinquencies and Net Credit Losses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 37,545		$ 48,444
Net credit losses (recoveries)	40	$ (520)
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	200		300
On-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	32,756		47,881
Net credit losses (recoveries)	40	(520)
Off-balance sheet [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,789		$ 563
Net credit losses (recoveries)	$ 0	$ 0