General – Cumulative Effect of Changes for Adoption of ASU 2014-09 (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Other current assets	$ 102,829	$ 71,656	$ 72,323
Total Current Assets	1,576,545	1,482,686	1,483,353
Deferred income taxes	1,071	2,148	813
Other long-term assets	51,391	21,234	22,567
Total Assets	3,048,515	2,956,569	2,957,234
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	209,012	191,453	193,635
Total Current Liabilities	918,571	890,121	892,303
Deferred credits and other long-term liabilities	34,959	24,844	19,100
Total Liabilities	1,716,764	1,684,302	1,680,740
Retained earnings	1,447,416	1,395,268	1,399,495
Total Stockholders' Equity	1,331,751	1,272,267	1,276,494
Total Liabilities and Stockholders' Equity	3,048,515	2,956,569	$ 2,957,234
Adjustments | ASU 2014-09
Assets
Other current assets	1,116	(667)
Total Current Assets	(11,003)	(667)
Deferred income taxes		1,335
Other long-term assets	4,667	(1,333)
Total Assets	(6,336)	(665)
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	(3,377)	(2,182)
Total Current Liabilities	(3,258)	(2,182)
Deferred credits and other long-term liabilities	(7,380)	5,744
Total Liabilities	(10,638)	3,562
Retained earnings	4,302	(4,227)
Total Stockholders' Equity	4,302	(4,227)
Total Liabilities and Stockholders' Equity	$ (6,336)	$ (665)