Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	CAPITAL SURPLUS [Member]	RETAINED EARNINGS [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME [Member]
Balance at beginning of period, Shares at Dec. 31, 2016		31,621,870
Balance at beginning of period at Dec. 31, 2016		$ 31,622	$ 101,730	$ 577,648	$ 94
Shares issued for stock options, Shares		272,693
Shares issued for stock options, Value		$ 273	4,571
Net income	$ 86,439			86,439
Net change	$ (2,421)				(2,421)
Dividends on common stock				(25,507)
Stock-based compensation arrangements			1,180
Balance at end of period, Shares at Dec. 31, 2017	31,894,563	31,894,563
Balance at end of period at Dec. 31, 2017	$ 775,629	$ 31,895	107,481	638,580	(2,327)
Shares issued for stock options, Shares		127,816
Shares issued for stock options, Value		$ 127	2,111
Shares issued for acquisitions, Shares		732,811
Shares issued for acquisitions, Value		$ 733	38,765
Net income	125,814			125,814
Net change	$ (430)				(430)
Dividends on common stock				(33,398)
Stock-based compensation arrangements			1,352
Shares acquired and canceled, Shares		(151,264)
Shares acquired and canceled, Value		$ (151)		(7,763)
Cumulative effect of change in accounting principle				(618)	618
Balance at end of period, Shares at Dec. 31, 2018	32,603,926	32,603,926
Balance at end of period at Dec. 31, 2018	$ 902,789	$ 32,604	149,709	722,615	(2,139)
Shares issued for stock options, Shares		117,012
Shares issued for stock options, Value		$ 117	2,367
Net income	134,879			134,879
Net change	$ 5,593				5,593
Dividends on common stock				(40,432)
Stock-based compensation arrangements			1,277
Shares acquired and canceled, Shares		(26,670)
Shares acquired and canceled, Value		$ (27)		(1,574)
Balance at end of period, Shares at Dec. 31, 2019	32,694,268	32,694,268
Balance at end of period at Dec. 31, 2019	$ 1,004,989	$ 32,694	$ 153,353	$ 815,488	$ 3,454