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)		120,919			87,002	33,917
Dividends		(25,453)			(25,453)
Purchase of treasury stock		(4,279)					(4,279)
Issuance of equity awards		472		(3,929)			4,401
Recognition of equity-based compensation		6,243		6,243
Sale of treasury stock		565		321			244
Exercise of stock options		10,268		1,844			8,424
Ending Balance at Jun. 30, 2017		2,071,119	55,057	1,037,898	1,204,436	(23,625)	(202,647)
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)		28,776			112,210	(83,434)
Reclassification of certain tax effects	[1]	12,917			12,917
Dividends		(29,040)			(29,040)
Purchase of treasury stock		(6,211)					(6,211)
Issuance of equity awards		494		(2,457)			2,951
Recognition of equity-based compensation		5,201		5,201
Sale of treasury stock		553		278			275
Exercise of stock options		7,446		1,543			5,903
Cumulative effect adjustment | ASU Nos. 2016-01 and 2017-12 [Member]	[2]	145			145
Ending Balance at Jun. 30, 2018		$ 2,201,812	$ 55,057	$ 1,050,660	$ 1,434,342	$ (128,959)	$ (209,288)

[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.