Fair Value Measurements - By Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and cash equivalents	$ 160,652	$ 353,272
Securities held to maturity	611,302	441,052
Portfolio loans, net	5,570,998	5,465,918
Financial liabilities:
Short-term borrowings	200,000	220,000
Junior subordinated debt owed to unconsolidated trusts	71,118	71,008
Level 1 | Carrying Amount
Financial assets:
Cash and cash equivalents	160,652	353,272
Level 1 | Fair Value
Financial assets:
Cash and cash equivalents	160,652	353,272
Level 2 | Carrying Amount
Financial assets:
Securities held to maturity	626,250	443,550
Accrued interest receivable	25,321	22,591
Financial liabilities:
Time deposits	1,552,283
Deposits		6,125,965
Securities sold under agreements to repurchase	255,906	304,566
Short-term borrowings	200,000	220,000
Long-term debt	50,000	50,000
Junior subordinated debt owed to unconsolidated trusts	71,118	71,008
Accrued interest payable	6,731	2,581
Level 2 | Fair Value
Financial assets:
Securities held to maturity	611,302	441,052
Accrued interest receivable	25,321	22,591
Financial liabilities:
Time deposits	1,536,729
Deposits		6,119,135
Securities sold under agreements to repurchase	255,906	304,566
Short-term borrowings	200,000	220,000
Long-term debt	49,562	50,000
Junior subordinated debt owed to unconsolidated trusts	71,118	71,008
Accrued interest payable	6,731	2,581
Level 3 | Carrying Amount
Financial assets:
Portfolio loans, net	5,570,998	5,465,918
Mortgage servicing rights	3,456	3,680
Other servicing rights	616	280
Level 3 | Fair Value
Financial assets:
Portfolio loans, net	5,424,280	5,361,406
Mortgage servicing rights	11,147	8,635
Other servicing rights	1,236	901
Level 3 | Senior notes | Carrying Amount
Financial liabilities:
Long-term debt	39,505	39,404
Level 3 | Senior notes | Fair Value
Financial liabilities:
Long-term debt	38,382	39,104
Level 3 | Subordinated debt | Carrying Amount
Financial liabilities:
Long-term debt	59,121	64,715
Level 3 | Subordinated debt | Fair Value
Financial liabilities:
Long-term debt	$ 57,470	$ 64,350