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		241	4,056
Stock-based compensation arrangements			819
Net income	$ 66,942			66,942
Dividends on common stock				(18,808)
Net change	70				70
Balance at end of period at Sep. 30, 2017	764,414	31,863	106,605	625,782	164
Balance at beginning of period at Jun. 30, 2017		31,818	105,440	610,758	446
Shares issued for stock options		45	811
Stock-based compensation arrangements			354
Net income	21,710			21,710
Dividends on common stock				(6,686)
Net change	(282)				(282)
Balance at end of period at Sep. 30, 2017	764,414	31,863	106,605	625,782	164
Balance at beginning of period at Dec. 31, 2017	775,629	31,895	107,481	638,580	(2,327)
Shares issued for stock options		122	2,015
Shares issued for acquisitions		733	38,765
Stock-based compensation arrangements			981
Net income	93,089			93,089
Cumulative effect of change in accounting principle				(618)	618
Dividends on common stock				(23,570)
Net change	(2,963)				(2,963)
Balance at end of period at Sep. 30, 2018	884,801	32,750	149,242	707,481	(4,672)
Balance at beginning of period at Jun. 30, 2018		32,731	148,494	684,425	(3,638)
Shares issued for stock options		19	395
Stock-based compensation arrangements			353
Net income	32,883			32,883
Dividends on common stock				(9,827)
Net change	(1,034)				(1,034)
Balance at end of period at Sep. 30, 2018	$ 884,801	$ 32,750	$ 149,242	$ 707,481	$ (4,672)