Borrowings - Narrative (Details)		12 Months Ended
	May 29, 2015 USD ($)	Dec. 31, 2017 USD ($) subsidiary	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Number of business trust subsidiaries, variable interest entities | subsidiary		3
Investment in trust preferred securities subsidiaries, common stock owned		$ 1,400,000
Subordinated debt		$ 45,000,000
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
Line of Credit Facility [Line Items]
Debt interest rate (in percent)	5.875%
Basis Spread on Variable Rate		3.63%
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%
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		3.10%
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%
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		1.89%
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%
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		1.40%
Subordinated Debt
Line of Credit Facility [Line Items]
Principal amount of subordinated debt		$ 121,393,000	$ 121,393,000
Subordinated Debt | 5.875% Subordinated Notes Due May 31, 2030
Line of Credit Facility [Line Items]
Face amount	$ 75,000,000
Debt interest rate (in percent)	5.875%
Notes price, percent of par value	100.00%
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
Line of Credit Facility [Line Items]
Principal amount of subordinated debt		75,000,000	75,000,000
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%
Line of Credit Facility [Line Items]
Principal amount of subordinated debt		15,464,000	15,464,000
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%
Line of Credit Facility [Line Items]
Principal amount of subordinated debt		10,310,000	10,310,000
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%
Line of Credit Facility [Line Items]
Principal amount of subordinated debt		20,619,000	20,619,000
LIBOR | Subordinated Debt | 5.875% Subordinated Notes Due May 31, 2030
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate	3.63%
Line of Credit
Line of Credit Facility [Line Items]
Remaining borrowing capacity		205,000,000	185,000,000
Federal Home Loan Bank Borrowings | Line of Credit
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		$ 490,700,000	331,500,000
Line of credit facility, expiration period		20 years
Federal Reserve Bank Advances
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		$ 236,500,000	248,500,000
Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value		$ 58,200,000	$ 37,800,000