General – Cumulative Effect of Changes for Adoption of ASU 2014-09 (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Other current assets	$ 119,164	$ 71,656	$ 72,323
Total Current Assets	1,668,149	1,482,686	1,483,353
Deferred income taxes	1,554	2,148	813
Other long-term assets	30,909	21,234	22,567
Total Assets	3,139,759	2,956,569	2,957,234
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	268,258	191,453	193,635
Total Current Liabilities	1,023,096	890,121	892,303
Deferred credits and other long-term liabilities	35,346	24,844	19,100
Total Liabilities	1,820,989	1,684,302	1,680,740
Retained earnings	1,420,271	1,395,268	1,399,495
Total Stockholders' Equity	1,318,770	1,272,267	1,276,494
Total Liabilities and Stockholders' Equity	3,139,759	2,956,569	$ 2,957,234
Adjustments | ASU 2014-09
Assets
Other current assets	(445)	(667)
Total Current Assets	(18,417)	(667)
Deferred income taxes		1,335
Other long-term assets	9,199	(1,333)
Total Assets	(9,218)	(665)
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	(11,127)	(2,182)
Total Current Liabilities	(11,077)	(2,182)
Deferred credits and other long-term liabilities	(6,835)	5,744
Total Liabilities	(17,912)	3,562
Retained earnings	8,694	(4,227)
Total Stockholders' Equity	8,694	(4,227)
Total Liabilities and Stockholders' Equity	$ (9,218)	$ (665)