CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Paid-In-Capital	Accumulated Other Comprehensive Income / (Loss)	Retained Earnings
Balance (in shares) at Dec. 31, 2016		217,792,000
Balance at Dec. 31, 2016	$ 568,545	$ 217,792	$ 77,452	$ (70,075)	$ 343,376
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	179,124				179,124
Other comprehensive income, net of tax
Pension liability adjustment	14,159			14,159
Foreign currency translation adjustments	9,960			9,960
Cash dividends	(122,017)				(122,017)
Stock compensation (in shares)		434,000
Stock compensation	12,399	$ 434	11,965
Employee stock buybacks (in shares)		(234,000)
Employee stock buybacks	(8,246)	$ (234)	(8,012)
Balance (in shares) at Dec. 31, 2017		217,992,000
Balance at Dec. 31, 2017	653,924	$ 217,992	81,405	(45,956)	400,483
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	114,067				114,067
Other comprehensive income, net of tax
Foreign currency translation adjustments	(8,070)			(8,070)
Cash dividends	(61,142)				(61,142)
Stock compensation (in shares)		411,000
Stock compensation	6,370	$ 411	5,959
Employee stock buybacks (in shares)		(186,000)
Employee stock buybacks	(9,338)	$ (186)	(9,152)
Balance (in shares) at Jun. 30, 2018		218,217,000
Balance at Jun. 30, 2018	$ 695,811	$ 218,217	$ 78,212	$ (54,026)	$ 453,408