Investment Securities - Narrative (Details)	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($) purchase_agreement	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) purchase_agreement
Investment Securities
Number of inverse putable reverse repurchase of the Bank's secured by collateral | purchase_agreement					3		3
Value of inverse putable reverse repurchases secured by collateral	$ 28,500,000				$ 28,500,000		$ 28,500,000
Other-than-temporary-impairment recovery/(loss) on securities		$ 0
Book value	703,083,000				703,083,000		380,963,000
Accumulated other comprehensive income (loss) before tax amount	5,500,000				5,500,000		(4,699,000)
Accumulated other comprehensive income, net of tax	3,260,000				3,260,000		(2,721,000)
Gross gains	2,100,000		$ 532,000		2,100,000	$ 1,300,000
Gross losses	50,000				50,000	9,000
Proceeds from sale or maturity of securities available for sale	215,000,000		$ 21,000,000		215,000,000	$ 207,000,000
FHLB stock	17,300,000				17,300,000
FRB stock	25,000,000				25,000,000
Other stock	14,400,000				14,400,000
Realized OTTI losses		$ 0
Amount of stock repurchased by FHLB	5,000,000
HOA
Investment Securities
Value of inverse putable reverse repurchases secured by collateral	20,900,000				20,900,000		21,500,000
Mortgage-backed securities
Investment Securities
Estimated par value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	56,000,000				56,000,000		63,600,000
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	$ 58,000,000				$ 58,000,000		$ 65,300,000
CRA
Investment Securities
Other-than-temporary-impairment recovery/(loss) on securities				$ (207,000)
Par value				50
Book value				$ 500,000