CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Maximum Cash Obligation Related To ESOP Shares [Member]
Beginning Balance at Dec. 31, 2017	$ 311,716	$ 48,930	$ 341,558	$ (2,446)	$ (33,018)	$ (43,308)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of 4,311, 2,327, 93,254 and 86,491 shares of common stock for 3 months ended June 30, 2018 and 2019, and the 9 months ended June 30, 2018 and 2019, respectively	5,052	2,721			2,331
Issuance 2,326, 2,106, 4,283, and 4,035 shares of common stock under the employee stock purchase plan for 3 months ended June 30, 2018 and 2019, and the 6 months ended June 30, 2018 and 2019, respectively	214	214
Unearned restricted stock compensation	165	165
Forfeiture of 2,554, 4,418, 2,762 and 5,121 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(131)	(131)
Change related to ESOP shares	(2,836)					(2,836)
Net income	20,921		20,921
Cash dividends ($0.75 and $0.82 per share for 6 months ended June 30, 2018 and 2019, respectively)	(7,002)		(7,002)
Reclassification of stranded tax effects due to the Tax Cuts and Jobs Act			526	(526)
Purchase of 8,831, 5,626, 56,263 and 75,397 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(3,199)				(3,199)
Other comprehensive income (loss)	(752)			(752)
Ending Balance at Jun. 30, 2018	324,148	51,899	356,003	(3,724)	(33,886)	(46,144)
Beginning Balance at Mar. 31, 2018	314,518	51,720	345,940	(3,875)	(33,488)	(45,779)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of 4,311, 2,327, 93,254 and 86,491 shares of common stock for 3 months ended June 30, 2018 and 2019, and the 9 months ended June 30, 2018 and 2019, respectively	248	141			107
Issuance 2,326, 2,106, 4,283, and 4,035 shares of common stock under the employee stock purchase plan for 3 months ended June 30, 2018 and 2019, and the 6 months ended June 30, 2018 and 2019, respectively	114	114
Unearned restricted stock compensation	47	47
Forfeiture of 2,554, 4,418, 2,762 and 5,121 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(123)	(123)
Change related to ESOP shares	(365)					(365)
Net income	10,063		10,063
Purchase of 8,831, 5,626, 56,263 and 75,397 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(505)				(505)
Other comprehensive income (loss)	151			151
Ending Balance at Jun. 30, 2018	324,148	51,899	356,003	(3,724)	(33,886)	(46,144)
Beginning Balance at Dec. 31, 2018	334,882	52,122	371,848	(3,250)	(36,968)	(48,870)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of 4,311, 2,327, 93,254 and 86,491 shares of common stock for 3 months ended June 30, 2018 and 2019, and the 9 months ended June 30, 2018 and 2019, respectively	5,279	3,016			2,263
Issuance 2,326, 2,106, 4,283, and 4,035 shares of common stock under the employee stock purchase plan for 3 months ended June 30, 2018 and 2019, and the 6 months ended June 30, 2018 and 2019, respectively	223	223
Unearned restricted stock compensation	407	407
Forfeiture of 2,554, 4,418, 2,762 and 5,121 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(254)	(254)
Share-based compensation	1	1
Change related to ESOP shares	(1,355)					(1,355)
Net income	22,423		22,423
Cash dividends ($0.75 and $0.82 per share for 6 months ended June 30, 2018 and 2019, respectively)	(7,657)		(7,657)
Purchase of 8,831, 5,626, 56,263 and 75,397 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(4,647)				(4,647)
Other comprehensive income (loss)	4,222			4,222
Ending Balance at Jun. 30, 2019	353,524	55,515	386,614	972	(39,352)	(50,225)
Beginning Balance at Mar. 31, 2019	340,780	55,295	375,394	(1,001)	(39,057)	(49,851)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of 4,311, 2,327, 93,254 and 86,491 shares of common stock for 3 months ended June 30, 2018 and 2019, and the 9 months ended June 30, 2018 and 2019, respectively	145	84			61
Issuance 2,326, 2,106, 4,283, and 4,035 shares of common stock under the employee stock purchase plan for 3 months ended June 30, 2018 and 2019, and the 6 months ended June 30, 2018 and 2019, respectively	118	118
Unearned restricted stock compensation	237	237
Forfeiture of 2,554, 4,418, 2,762 and 5,121 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(220)	(220)
Share-based compensation	1	1
Change related to ESOP shares	(374)					(374)
Net income	11,220		11,220
Purchase of 8,831, 5,626, 56,263 and 75,397 shares of common stock for 3 months ended June 30, 2018 and 2019 and the 6 months ended June 30, 2018 and 2019, respectively	(356)				(356)
Other comprehensive income (loss)	1,973			1,973
Ending Balance at Jun. 30, 2019	$ 353,524	$ 55,515	$ 386,614	$ 972	$ (39,352)	$ (50,225)