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, 2016		$ 31,622	$ 101,730	$ 577,648	$ 94
Shares issued for stock options		196	3,245
Stock-based compensation arrangements			465
Net income	$ 45,232			45,232
Dividends on common stock				(12,122)
Net change	352				352
Balance at end of period at Jun. 30, 2017	748,462	31,818	105,440	610,758	446
Balance at beginning of period at Mar. 31, 2017		31,782	102,605	593,631	(84)
Shares issued for stock options		36	2,592
Stock-based compensation arrangements			243
Net income	23,182			23,182
Dividends on common stock				(6,055)
Net change	530				530
Balance at end of period at Jun. 30, 2017	748,462	31,818	105,440	610,758	446
Balance at beginning of period at Dec. 31, 2017	775,629	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)