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Fair Value (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value on a Recurring Basis [Member]    
Financial Assets (Liabilities) [Abstract]    
Cash equivalents and current investments $ 36,531 $ 50,307
Other long-term assets 3,726 2,782
Forward contracts 158 1,371
Life insurance contracts 37,737 32,286
Financial assets (liabilities), net 78,152 86,746
Restricted current investments 11,800 10,900
Fair Value on a Recurring Basis [Member] | Level 1 [Member]    
Financial Assets (Liabilities) [Abstract]    
Cash equivalents and current investments 36,531 50,307
Other long-term assets 3,726 2,782
Forward contracts 0 0
Life insurance contracts 0 0
Financial assets (liabilities), net 40,257 53,089
Fair Value on a Recurring Basis [Member] | Level 2 [Member]    
Financial Assets (Liabilities) [Abstract]    
Cash equivalents and current investments 0 0
Other long-term assets 0 0
Forward contracts 158 1,371
Life insurance contracts 0 0
Financial assets (liabilities), net 158 1,371
Fair Value on a Recurring Basis [Member] | Level 3 [Member]    
Financial Assets (Liabilities) [Abstract]    
Cash equivalents and current investments 0 0
Other long-term assets 0 0
Forward contracts 0 0
Life insurance contracts 37,737 32,286
Financial assets (liabilities), net 37,737 32,286
Insurance Company Contracts [Member]    
Changes in Fair Value of Level 3 Marketable Securities [Roll Forward]    
Beginning balance 32,287 27,292
Actual Return on Plan Assets [Abstract]    
Relating to assets still held at the reporting date 4,917 2,196
Purchases and issuances 895 3,051
Sales and settlements (362) (252)
Transfers into Level 3 0 0
Ending balance $ 37,737 $ 32,287