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Summary of Significant Accounting Policies (Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract]    
Foreign currency translation $ (385,627) $ (283,647)
Unrealized gains on investments 161 1,211
Unrealized losses on hedging activities (295)  
Estimated tax loss on unrealized gains on investments (19) (415)
Accumulated other comprehensive loss $ (385,780) $ (282,851)