CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2016		17,171,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2016	$ 390,804	$ 1,073	$ 115,123	$ 294,365	$ (19,757)
Net income	37,943			37,943
Total other comprehensive income (loss), net of tax	3,386				3,386
Cash dividends declared	(19,974)			(19,974)
Share-based compensation	1,872		1,872
Exercise of stock options and issuance of other compensation-related equity awards, shares		43,000
Exercise of stock options and issuance of other compensation-related equity awards, value	197	$ 3	194
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2017		17,214,000
Shareholders' Equity, Ending Balance at Sep. 30, 2017	$ 414,228	$ 1,076	117,189	312,334	(16,371)
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2017	17,226,508	17,227,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2017	$ 413,284	$ 1,077	117,961	317,756	(23,510)
Net income	51,396			51,396
Total other comprehensive income (loss), net of tax	(15,567)				(15,567)
Cash dividends declared	(22,467)			(22,467)
Share-based compensation	1,977		1,977
Exercise of stock options and issuance of other compensation-related equity awards, shares		63,000
Exercise of stock options and issuance of other compensation-related equity awards, value	$ (714)	$ 4	(718)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2018	17,290,443	17,290,000
Shareholders' Equity, Ending Balance at Sep. 30, 2018	$ 427,909	$ 1,081	$ 119,220	$ 346,685	$ (39,077)