Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	COMMON STOCK [Member]	CAPITAL SURPLUS [Member]	RETAINED EARNINGS [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) [Member]
Balance at beginning of period at Dec. 31, 2017		$ 31,895	$ 107,481	$ 638,580	$ (2,327)
Shares issued for stock options		103	1,620
Shares issued for acquisitions		733	38,765
Stock-based compensation arrangements			628
Net income	$ 60,206			60,206
Cumulative effect of change in accounting principle				(618)	618
Dividends on common stock				(13,743)
Net change	(1,929)				(1,929)
Balance at end of period at Jun. 30, 2018	862,012	32,731	148,494	684,425	(3,638)
Balance at beginning of period at Mar. 31, 2018		32,708	147,762	660,723	(3,097)
Shares issued for stock options		23	410
Stock-based compensation arrangements			322
Net income	30,586			30,586
Dividends on common stock				(6,884)
Net change	(541)				(541)
Balance at end of period at Jun. 30, 2018	862,012	32,731	148,494	684,425	(3,638)
Balance at beginning of period at Dec. 31, 2018	902,789	32,604	149,709	722,615	(2,139)
Shares issued for stock options		36	745
Stock-based compensation arrangements			541
Net income	66,004			66,004
Dividends on common stock				(19,529)
Net change	5,794				5,794
Balance at end of period at Jun. 30, 2019	956,380	32,640	150,995	769,090	3,655
Balance at beginning of period at Mar. 31, 2019		32,618	150,195	744,713	401
Shares issued for stock options		22	433
Stock-based compensation arrangements			367
Net income	34,167			34,167
Dividends on common stock				(9,790)
Net change	3,254				3,254
Balance at end of period at Jun. 30, 2019	$ 956,380	$ 32,640	$ 150,995	$ 769,090	$ 3,655