Consolidated Statements of Changes in Stockholders' Equity (unaudited) - USD ($)	Total	Common Stock	Additional paid-in capital	Unearned compensation- ESOP	Retained earnings	Accumulated other comprehensive (loss) income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle ASU 2016-01 (Note 2)			$ 27,000		$ (27,000)
Beginning balance at Dec. 31, 2016	$ 386,907,000	$ 268,000	249,308,000	$ (20,496,000)	161,896,000	$ (4,069,000)
Beginning balance (shares) at Dec. 31, 2016		26,759,953
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	9,411,000				7,487,000	1,924,000
ESOP shares committed to be released	341,000		151,000	190,000
Common stock dividends declared ($0.08 and $0.45 per common share for the nine months ended September 30, 2016 and 2017, respectively)	(1,196,000)				(1,196,000)
Restricted stock awards granted (shares)		175,695
Restricted stock awards granted	1,000
Restricted stock awards forfeited (shares)		(84,417)
Restricted stock awards forfeited	(1,000)
Share-based compensation expense	1,408,000		1,408,000
Share redemption for tax withholdings for restricted stock vesting (shares)		(5,903)
Share redemption for tax withholdings for restricted stock vesting	(107,000)		(107,000)
Proceeds from exercise of options (shares)		13,000
Proceeds from exercise of options	180,000		180,000
Ending balance at Mar. 31, 2017	396,944,000	$ 268,000	250,967,000	(20,306,000)	168,160,000	(2,145,000)
Ending balance (shares) at Mar. 31, 2017		26,858,328
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle ASU 2016-01 (Note 2) | ASU 2016-01					173,000	(173,000)
Cumulative effect of change in accounting principle ASU 2016-01 (Note 2) | ASU 2018-02					279,000	(279,000)
Beginning balance at Dec. 31, 2017	397,806,000	$ 268,000	254,750,000	(19,737,000)	163,978,000	(1,453,000)
Beginning balance (shares) at Dec. 31, 2017		26,827,660
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	6,617,000				6,583,000	34,000
ESOP shares committed to be released	378,000		188,000	190,000
Common stock dividends declared ($0.08 and $0.45 per common share for the nine months ended September 30, 2016 and 2017, respectively)	(10,889,000)				(10,889,000)
Restricted stock awards granted (shares)		36,500
Restricted stock awards forfeited (shares)		(23,000)
Share-based compensation expense	1,349,000		1,349,000
Share redemption for tax withholdings for restricted stock vesting (shares)		(15,969)
Share redemption for tax withholdings for restricted stock vesting	$ (325,000)		(325,000)
Proceeds from exercise of options (shares)	36,330	36,330
Proceeds from exercise of options	$ 508,000		508,000
Ending balance at Mar. 31, 2018	$ 395,444,000	$ 268,000	$ 256,470,000	$ (19,547,000)	$ 160,124,000	$ (1,871,000)
Ending balance (shares) at Mar. 31, 2018		26,861,521