Investment Securities - Narrative (Details)	3 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($) purchase_agreement	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) purchase_agreement
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ 12,600,000	$ (646,000)		$ (646,000)
Accumulated other comprehensive income, net of tax	$ 8,851,000	(415,000)		$ (415,000)
Number of inverse putable reverse repurchase of the Bank's secured by collateral | purchase_agreement	2			3
Value of inverse putable reverse repurchases secured by collateral	$ 18,500,000	28,500,000		$ 28,500,000
Other-than-temporary-impairment recovery/(loss) on securities	0	0
Book value	863,243,000	787,429,000		787,429,000
Gross gains	6,000	0	$ 0
Gross losses	0	(252,000)	0
Proceeds from sale or maturity of securities available for sale		14,000,000
FHLB stock	17,300,000
FRB stock	33,200,000
Other stock	31,700,000
Amount of stock repurchased by FHLB	0		$ 0
HOA
Investment Securities
Value of inverse putable reverse repurchases secured by collateral	25,400,000	17,600,000		17,600,000
Mortgage-backed securities: residential
Investment Securities
Estimated par value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	59,700,000	55,600,000		55,600,000
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	$ 60,100,000	$ 57,000,000		$ 57,000,000