Fair Value Measurements - By Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and cash equivalents	$ 230,730	$ 353,272
Securities held to maturity	496,715	441,052
Portfolio loans, net	5,501,982	5,465,918
Financial liabilities:
Short-term borrowings	150,000	220,000
Junior subordinated debt owed to unconsolidated trusts	71,081	71,008
Level 1 | Carrying Amount
Financial assets:
Cash and cash equivalents	230,730	353,272
Level 1 | Fair Value
Financial assets:
Cash and cash equivalents	230,730	353,272
Level 2 | Carrying Amount
Financial assets:
Securities held to maturity	507,780	443,550
Accrued interest receivable	22,476	22,591
Financial liabilities:
Time deposits	1,474,506
Deposits		6,125,965
Securities sold under agreements to repurchase	240,109	304,566
Short-term borrowings	150,000	220,000
Long-term debt	50,000	50,000
Junior subordinated debt owed to unconsolidated trusts	71,081	71,008
Accrued interest payable	4,198	2,581
Level 2 | Fair Value
Financial assets:
Securities held to maturity	496,715	441,052
Accrued interest receivable	22,476	22,591
Financial liabilities:
Time deposits	1,461,928
Deposits		6,119,135
Securities sold under agreements to repurchase	240,109	304,566
Short-term borrowings	150,000	220,000
Long-term debt	50,000	50,000
Junior subordinated debt owed to unconsolidated trusts	71,081	71,008
Accrued interest payable	4,198	2,581
Level 3 | Carrying Amount
Financial assets:
Portfolio loans, net	5,501,982	5,465,918
Mortgage servicing rights	3,502	3,680
Other servicing rights	390	280
Level 3 | Fair Value
Financial assets:
Portfolio loans, net	5,423,322	5,361,406
Mortgage servicing rights	10,813	8,635
Other servicing rights	1,041	901
Level 3 | Senior notes | Carrying Amount
Financial liabilities:
Long-term debt	39,472	39,404
Level 3 | Senior notes | Fair Value
Financial liabilities:
Long-term debt	38,328	39,104
Level 3 | Subordinated debt | Carrying Amount
Financial liabilities:
Long-term debt	64,653	64,715
Level 3 | Subordinated debt | Fair Value
Financial liabilities:
Long-term debt	$ 63,103	$ 64,350