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Summary of Significant Accounting Policies - Comprehensive Income (Loss) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Accumulated other comprehensive loss consists of foreign currency translation adjustments, net of tax $ 33,270,000 $ 55,262,000  
Net unrealized losses on available-for-sale investments, net of tax 58,000 68,000  
Losses on currency swaps, net of gains on long-term intercompany loans 1,271,000 1,271,000  
Net realized gains reclassified into current operations 829,000 191,000 $ 344,000
Accumulated Other Comprehensive Loss [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Net realized gains reclassified into current operations 829,000 191,000 344,000
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Net unrealized gains on derivative instruments $ 0 $ (37,000) $ (206,000)