Fair Value Measurements - Segregated by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 330,407	$ 239,973
Securities held to maturity	15,975	603,360
Portfolio loans, net	6,464,166	5,517,780
Financial liabilities:
Short-term borrowings	30,739
Junior subordinated debt owed to unconsolidated trusts	71,192	71,155
Level 1 | Carrying Amount
Financial assets:
Cash and cash equivalents	330,407	239,973
Level 1 | Fair Value
Financial assets:
Cash and cash equivalents	330,407	239,973
Level 2 | Carrying Amount
Financial assets:
Securities held to maturity	15,846	608,660
Accrued interest receivable	29,748	22,314
Financial liabilities:
Time deposits	1,757,078	1,497,003
Securities sold under agreements to repurchase	217,077	185,796
Short-term borrowings	30,739
Long-term debt	89,476	50,000
Junior subordinated debt owed to unconsolidated trusts	71,192	71,155
Accrued interest payable	7,589	6,568
Level 2 | Fair Value
Financial assets:
Securities held to maturity	15,975	603,360
Accrued interest receivable	29,748	22,314
Financial liabilities:
Time deposits	1,746,252	1,482,301
Securities sold under agreements to repurchase	217,077	185,796
Short-term borrowings	30,871
Long-term debt	89,738	49,873
Junior subordinated debt owed to unconsolidated trusts	65,152	65,182
Accrued interest payable	7,589	6,568
Level 3 | Carrying Amount
Financial assets:
Portfolio loans, net	6,464,166	5,517,780
Mortgage servicing rights	10,201	3,315
Other servicing rights	839	781
Level 3 | Fair Value
Financial assets:
Portfolio loans, net	6,402,404	5,473,063
Mortgage servicing rights	16,194	11,051
Other servicing rights	1,413	1,443
Level 3 | Senior notes | Carrying Amount
Financial liabilities:
Long-term debt	39,573	39,539
Level 3 | Senior notes | Fair Value
Financial liabilities:
Long-term debt	39,797	39,452
Level 3 | Subordinated debt | Carrying Amount
Financial liabilities:
Long-term debt	59,172	59,147
Level 3 | Subordinated debt | Fair Value
Financial liabilities:
Long-term debt	$ 58,748	$ 58,186