CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock Outstanding [Member]	Common Stock Amount Issued [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, 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)
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 (in shares)								0
Treasury stock purchased						(3,306)	3,306	$ 0
Treasury stock purchased (in shares)	(58,491)
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle - equity securities				208	(208)			0
Net income				168,641				168,641
Other comprehensive income (loss), net of tax					(41,398)			(41,398)
Dividends declared:
Common				(73,843)				(73,843)
Common stock issued under employee stock plan		313	6,130					6,443
Common stock issued under employee stock plan (in shares)	312,998
Stock-based compensation			6,064					6,064
Distribution of stock under deferred compensation arrangements						1,898	(1,898)	$ 0
Distribution of stock under deferred compensation arrangements (in shares)	35,233
Treasury stock purchased (in shares)								0
Treasury stock purchased						(298)	298	$ 0
Treasury stock purchased (in shares)	(5,142)
Treasury stock issued to benefit plan			4,675			7,886	0	12,561
Treasury stock issued to benefit plan (in shares)	218,658
Balance at Dec. 31, 2018		$ 51,577	$ 911,748	$ 795,563	$ (45,305)	$ (11,528)	$ 11,728	$ 1,713,783
Balance (in shares) at Dec. 31, 2018	51,257,824