CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income/(Loss) [Member]	Treasury Stock [Member]	Deferred Compensation Arrangements [Member]	Total
Balance at Dec. 31, 2014	$ 41,606	$ 409,984	$ 525,985	$ 30,720	$ (20,391)	$ 0	$ 987,904
Balance (in shares) at Dec. 31, 2014	40,747,721
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			91,230				91,230
Other comprehensive income (loss), net of tax				(11,485)			(11,485)
Dividends declared:
Common			(50,624)				(50,624)
Common stock issued under employee stock plan	$ 459	9,315					9,774
Common stock issued under employee stock plan (in shares)	458,817
Stock-based compensation		4,201					4,201
Stock issued for acquisitions	$ 2,378	99,824					102,202
Stock issued for acquisitions (in shares)	2,377,329
Treasury stock purchased					(9,126)		$ (9,126)
Treasury stock purchased (in shares)	(265,230)						(300,000)
Treasury stock issued to benefit plan		4,691			11,880		$ 16,571
Treasury stock issued to benefit plan (in shares)	456,223
Balance at Dec. 31, 2015	$ 44,443	528,015	566,591	19,235	(17,637)	0	1,140,647
Balance (in shares) at Dec. 31, 2015	43,774,860
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			103,812				103,812
Other comprehensive income (loss), net of tax				(11,392)			(11,392)
Dividends declared:
Common			(55,711)				(55,711)
Common stock issued under employee stock plan	$ 507	10,036					10,543
Common stock issued under employee stock plan (in shares)	507,784
Stock-based compensation		4,783					4,783
Treasury stock purchased					(3,470)		$ (3,470)
Treasury stock purchased (in shares)	(67,826)						0
Treasury stock issued to benefit plan		2,941			5,947		$ 8,888
Treasury stock issued to benefit plan (in shares)	222,597
Balance at Dec. 31, 2016	$ 44,950	545,775	614,692	7,843	(15,160)	0	1,198,100
Balance (in shares) at Dec. 31, 2016	44,437,415
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			150,717				150,717
Other comprehensive income (loss), net of tax				(10,897)			(10,897)
Reclassification related to tax effect of Tax Cuts and Jobs Act			645	(645)			0
Dividends declared:
Common			(65,497)				(65,497)
Common stock issued under employee stock plan	$ 265	4,298					4,563
Common stock issued under employee stock plan (in shares)	264,640
Stock-based compensation		5,137					5,137
Stock issued for acquisitions	$ 6,049	337,083					343,132
Stock issued for acquisitions (in shares)	6,048,849
Deferred compensation arrangements acquired					(10,022)	10,022	0
Deferred compensation arrangements acquired (in shares)	(179,003)
Treasury stock purchased					(3,306)	3,306	$ 0
Treasury stock purchased (in shares)	(58,491)						0
Treasury stock issued to benefit plan		2,586			7,474	0	$ 10,060
Treasury stock issued to benefit plan (in shares)	182,667
Balance at Dec. 31, 2017	$ 51,264	$ 894,879	$ 700,557	$ (3,699)	$ (21,014)	$ 13,328	$ 1,635,315
Balance (in shares) at Dec. 31, 2017	50,696,077