Investment Securities - Narrative (Details)	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) purchase_agreement	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) purchase_agreement
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ 4,200,000		$ (4,700,000)			$ 4,200,000		$ (4,700,000)
Accumulated other comprehensive income, net of tax	2,476,000		(2,721,000)			$ 2,476,000		$ (2,721,000)
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		$ 28,500,000
Other-than-temporary-impairment recovery/(loss) on securities	0	$ 0		$ 2,000
Book value	703,944,000		380,963,000			703,944,000		380,963,000
Gross gains	897,000	2,100,000		512,000		3,000,000	$ 1,800,000
Gross losses	(1,000)	(50,000)		0		(51,000)	(9,000)
Proceeds from sale or maturity of securities available for sale	3,700,000	$ 215,000,000		16,600,000		243,000,000	$ 223,000,000
FHLB stock	17,300,000					17,300,000
FRB stock	25,000,000					25,000,000
Other stock	16,100,000					16,100,000
Amount of stock repurchased by FHLB			0	$ 0
HOA
Investment Securities
Value of inverse putable reverse repurchases secured by collateral	18,500,000		21,500,000			18,500,000		21,500,000
Mortgage-backed securities: residential
Investment Securities
Estimated par value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	62,400,000		63,600,000			62,400,000		63,600,000
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	$ 64,200,000		$ 65,300,000			$ 64,200,000		$ 65,300,000
CRA
Investment Securities
Other-than-temporary-impairment recovery/(loss) on securities					$ (207,000)
Par value					50
Book value					$ 500,000