Investment Securities - Narrative (Details)	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) purchase_agreement
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ (23,600,000)		$ 646,000		$ (23,600,000)		$ 646,000
Accumulated other comprehensive income, net of tax	(16,677,000)		415,000		(16,677,000)		$ 415,000
Number of inverse putable reverse repurchase of the Bank's secured by collateral | purchase_agreement							3
Value of inverse putable reverse repurchases secured by collateral			28,500,000				$ 28,500,000
Gross gains	1,300,000	$ 330,000		$ 897,000	1,600,000	$ 3,000,000
Gross losses	(208,000)			(1,000)	(208,000)	(51,000)
Proceeds from sale or maturity of securities available for sale	379,000,000	16,200,000		28,400,000	395,000,000	243,000,000
FHLB stock	23,200,000				23,200,000
FRB stock	51,400,000				51,400,000
Other stock	38,000,000				38,000,000
Amount of stock repurchased by FHLB	15,000,000		0			5,000,000
Other than temporary impairment losses	0	$ 0		$ 0	0	$ 0
Net loss on CRA equity investment	600,000
HOA
Investment Securities
Value of inverse putable reverse repurchases secured by collateral	2,300,000		17,600,000		2,300,000		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	21,700,000		55,600,000		21,700,000		55,600,000
Fair value of securities pledged as collateral for the Bank's inverse putable reverse repurchases	$ 22,200,000		$ 57,000,000		$ 22,200,000		$ 57,000,000