Investment Securities - Narrative (Details) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) purchase_agreement	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) purchase_agreement
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ (17,400)		$ 646		$ (17,400)		$ 646
Accumulated other comprehensive income, net of tax	(12,213)		415		$ (12,213)		$ 415
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		28,500		$ 18,500		$ 28,500
Gross gains	330	$ 6		$ 2,100	336	$ 2,100
Gross losses	0	0		(50)	0	(50)
Proceeds from sale or maturity of securities available for sale	16,200	$ 0		215,000	16,210	$ 215,000
FHLB stock	17,300				17,300
FRB stock	33,200				33,200
Other stock	32,200				32,200
Amount of stock repurchased by FHLB	0		0	$ 0
Other than temporary impairment losses					0
HOA
Investment Securities
Value of inverse putable reverse repurchases secured by collateral	23,000		17,600		23,000		17,600
Mortgage-backed securities: residential
Investment Securities
Estimated par value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	46,300		55,600		46,300		55,600
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	$ 47,100		$ 57,000		$ 47,100		$ 57,000