CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Total
Balance at Dec. 31, 2016		$ 412,726	$ 11,407	$ 126,462	$ (16,717)	$ (2,846)	$ 531,032
Net income				32,640			32,640
Other comprehensive loss, net of tax						17	17
Cash dividends declared on common stock ($0.225 per share)				(7,251)			(7,251)
Stock issuance costs		(180)					(180)
Exercise of stock options (102,378 and 10,846 shares)			118				118
Restricted stock grants (23,018 and 57,164 shares)
Stock-based compensation expense			1,315				1,315
Balance at Sep. 30, 2017		412,546	12,840	151,851	(16,717)	(2,829)	557,691
Balance at Dec. 31, 2017		412,546	13,602	160,025	(16,717)	(4,019)	565,437
Reclassification of stranded tax effects (ASU 2018-02) (see Note 8)				709		(709)
Cumulative effect of adopting ASU 2016-01 (see Note 8)				(55)		55
Net income				41,680			41,680
Other comprehensive loss, net of tax						(6,011)	(6,011)
Cash dividends declared on common stock ($0.225 per share)				(7,258)			(7,258)
Exercise of stock options (102,378 and 10,846 shares)			524				524
Restricted stock grants (23,018 and 57,164 shares)
Net performance units issued (42,672 shares)			(819)				(819)
Stock-based compensation expense			1,318				1,318
Balance at Sep. 30, 2018		$ 412,546	$ 14,625	$ 195,101	$ (16,717)	$ (10,684)	$ 594,871