Borrowings - Subordinated Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	May 29, 2015
Debt Instrument [Line Items]
Less unamortized debt issuance costs	$ 35,900	$ 35,500
Subordinated debt	$ 120,587	$ 120,454
Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 3.10%, with a rate of 4.77% and 4.10%
Debt Instrument [Line Items]
Debt maturity period (in years)	30 years
Description of Variable Rate Basis	three-month LIBOR
Basis Spread on Variable Rate	3.10%
Variable Rate	4.77%	4.10%
Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 1.89%, with a rate of 3.49% and 2.88%
Debt Instrument [Line Items]
Debt maturity period (in years)	30 years
Description of Variable Rate Basis	three-month LIBOR
Basis Spread on Variable Rate	1.89%
Variable Rate	3.49%	2.88%
Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 1.40%, with a rate of 2.99% and 2.36%
Debt Instrument [Line Items]
Debt maturity period (in years)	30 years
Description of Variable Rate Basis	three-month LIBOR
Basis Spread on Variable Rate	1.40%
Variable Rate	2.99%	2.36%
Subordinated notes at an annual fixed rate of 5.875% until May 31, 2025 with interest paid semiannually until June 1, 2025, thereafter, at a floating rate equal to three-month LIBOR plus 3.63% from June 1, 2025 to May 31, 2030, with interest paid quarterly until maturity
Debt Instrument [Line Items]
Description of Variable Rate Basis	three-month LIBOR
Basis Spread on Variable Rate	3.63%
Debt interest rate (in percent)			5.875%
Subordinated Debt
Debt Instrument [Line Items]
Principal amount of subordinated debt	$ 121,393	$ 121,393
Less unamortized debt issuance costs	806	939
Subordinated debt	120,587	120,454
Subordinated Debt | Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 3.10%, with a rate of 4.77% and 4.10%
Debt Instrument [Line Items]
Principal amount of subordinated debt	15,464	15,464
Subordinated Debt | Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 1.89%, with a rate of 3.49% and 2.88%
Debt Instrument [Line Items]
Principal amount of subordinated debt	10,310	10,310
Subordinated Debt | Floating rate 30-year capital securities with interest adjusted and paid quarterly at three-month LIBOR plus 1.40%, with a rate of 2.99% and 2.36%
Debt Instrument [Line Items]
Principal amount of subordinated debt	20,619	20,619
Subordinated Debt | Subordinated notes at an annual fixed rate of 5.875% until May 31, 2025 with interest paid semiannually until June 1, 2025, thereafter, at a floating rate equal to three-month LIBOR plus 3.63% from June 1, 2025 to May 31, 2030, with interest paid quarterly until maturity
Debt Instrument [Line Items]
Principal amount of subordinated debt	$ 75,000	$ 75,000