Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning Balance at Dec. 31, 2014		$ 1,643,758	$ 55,057	$ 894,602	$ 963,911	$ 11,006	$ (280,818)
Total comprehensive income		101,349			116,073	(14,724)
Dividends		(45,994)			(45,994)
Purchase of treasury stock		(8,457)					(8,457)
Issuance of equity awards		459		(3,278)			3,737
Recognition of equity based compensation		10,292		10,292
Net tax benefit related to equity compensation plans		944		944
Sale of treasury stock		1,056		611			445
Exercise of stock options		10,550		4,083			6,467
Common stock issuance for acquisition		179,737		112,635			67,102
Ending Balance at Dec. 31, 2015		1,893,694	55,057	1,019,889	1,033,990	(3,718)	(211,524)
Total comprehensive income		104,977			158,801	(53,824)
Dividends		(49,048)			(49,048)
Purchase of treasury stock		(16,367)					(16,367)
Issuance of equity awards		429		(3,011)			3,440
Recognition of equity based compensation		11,306		11,306
Sale of treasury stock		1,096		480			616
Exercise of stock options		15,815		3,417			12,398
Cumulative effect adjustment	[1]	482		1,338	(856)
Ending Balance at Dec. 31, 2016		1,962,384	55,057	1,033,419	1,142,887	(57,542)	(211,437)
Total comprehensive income		259,122			247,105	12,017
Dividends		(51,882)			(51,882)
Purchase of treasury stock		(15,276)					(15,276)
Issuance of equity awards		472		(2,871)			3,343
Recognition of equity based compensation		12,844		12,844
Sale of treasury stock		1,120		608			512
Exercise of stock options		12,747		2,095			10,652
Ending Balance at Dec. 31, 2017		$ 2,181,531	$ 55,057	$ 1,046,095	$ 1,338,110	$ (45,525)	$ (212,206)

[1] Related to the adoption of Accounting Standards Update No. 2016-09. See Note 2, New Accounting Pronouncements, for further detail.