Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock
Beginning Balance at Dec. 31, 2016		$ 1,962,384	$ 55,057	$ 1,033,419	$ 1,142,887	$ (57,542)	$ (211,437)
Total comprehensive income (loss)		170,018			135,144	34,874
Dividends		(38,166)			(38,166)
Purchase of treasury stock		(14,369)					(14,369)
Issuance of equity awards		471		(3,364)			3,835
Recognition of equity-based compensation		9,576		9,576
Sale of treasury stock		849		468			381
Exercise of stock options		10,780		1,923			8,857
Ending Balance at Sep. 30, 2017		2,101,543	55,057	1,042,022	1,239,865	(22,668)	(212,733)
Beginning Balance at Dec. 31, 2017		2,181,531	55,057	1,046,095	1,338,110	(45,525)	(212,206)
Total comprehensive income (loss)		59,943			170,059	(110,116)
Reclassification of certain tax effects	[1]	12,917			12,917
Dividends		(43,499)			(43,499)
Purchase of treasury stock		(26,417)					(26,417)
Issuance of equity awards		495		(2,004)			2,499
Recognition of equity-based compensation		8,469		8,469
Sale of treasury stock		815		409			406
Exercise of stock options		9,065		1,832			7,233
Cumulative effect adjustment | ASU Nos. 2016-01 and 2017-12 [Member]	[2]	145			145
Ending Balance at Sep. 30, 2018		$ 2,203,464	$ 55,057	$ 1,054,801	$ 1,477,732	$ (155,641)	$ (228,485)

[1] Related to the adoption of ASU No. 2018-02. See Note 3, “New Accounting Pronouncements,” for further detail.

[2] Related to the adoption of ASU Nos. 2016-01 and 2017-12. See Note 3, “New Accounting Pronouncements,” for further detail.