Fair Value Measurements - By Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Financial assets:
Cash and cash equivalents	$ 214,381	$ 166,706	$ 360,835	$ 319,280
Securities held to maturity	282,488	47,683
Portfolio loans, net	5,034,829	3,831,105
Financial liabilities:
Short-term borrowings	212,850	75,000
Junior subordinated debt owed to unconsolidated trusts	70,973	70,868
Level 1 | Carrying Amount
Financial assets:
Cash and cash equivalents	214,381	166,706
Level 1 | Fair Value
Financial assets:
Cash and cash equivalents	214,381	166,706
Level 2 | Carrying Amount
Financial assets:
Securities held to maturity	281,975	47,820
Accrued interest receivable	19,780	15,562
Financial liabilities:
Deposits	5,373,283	4,374,298
Securities sold under agreements to repurchase	219,071	189,157
Short-term borrowings	212,850	75,000
Long-term debt	50,000	80,000
Junior subordinated debt owed to unconsolidated trusts	70,973	70,868
Accrued interest payable	3,122	987
Level 2 | Fair Value
Financial assets:
Securities held to maturity	282,488	47,683
Accrued interest receivable	19,780	15,562
Financial liabilities:
Deposits	5,370,642	4,368,891
Securities sold under agreements to repurchase	219,071	189,157
Short-term borrowings	212,850	75,000
Long-term debt	50,000	80,000
Junior subordinated debt owed to unconsolidated trusts	70,973	70,868
Accrued interest payable	3,122	987
Level 3 | Carrying Amount
Financial assets:
Portfolio loans, net	5,034,829	3,831,105
Mortgage servicing rights	3,168	3,074
Other servicing rights	204
Level 3 | Fair Value
Financial assets:
Portfolio loans, net	5,043,585	3,841,760
Mortgage servicing rights	7,314	$ 7,803
Other servicing rights	921
Level 3 | Senior notes | Carrying Amount
Financial liabilities:
Long-term debt	39,370
Level 3 | Senior notes | Fair Value
Financial liabilities:
Long-term debt	39,776
Level 3 | Subordinated debt | Carrying Amount
Financial liabilities:
Long-term debt	64,745
Level 3 | Subordinated debt | Fair Value
Financial liabilities:
Long-term debt	$ 66,186