CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive income/ (loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2016	$ 217,792	$ 77,452	$ (70,075)	$ 343,376	$ 568,545
Balance (in shares) at Dec. 31, 2016	217,792,000
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	$ 434	11,965			12,399
Stock compensation (in shares)	434,000
Employee stock buybacks	$ (234)	(8,012)			(8,246)
Employee stock buybacks (in shares)	(234,000)
Balance at Dec. 31, 2017	$ 217,992	81,405	(45,956)	400,483	653,924
Balance (in shares) at Dec. 31, 2017	217,992,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				180,695	180,695
Other comprehensive income, net of tax
Foreign currency translation adjustments			(8,681)		(8,681)
Cash dividends				(91,677)	(91,677)
Stock compensation	$ 409	9,621			10,030
Stock compensation (in shares)	409,000
Employee stock buybacks	$ (189)	(9,344)			(9,533)
Employee stock buybacks (in shares)	(189,000)
Balance at Sep. 30, 2018	$ 218,212	$ 81,682	$ (54,637)	$ 489,501	$ 734,758
Balance (in shares) at Sep. 30, 2018	218,212,000