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Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 85,423 $ 78,687 $ 101,736 $ 138,285
Cash equivalents 37,600 37,200    
Domestic Bank Accounts        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 57,706 62,495    
Foreign Bank Accounts        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 27,717 16,192    
Domestic Joint Ventures        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 12,337 8,544    
Foreign Joint Ventures        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 11 441    
Investments in Joint Ventures        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 12,348 8,985    
Cash Not Held by Joint Ventures        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 73,075 $ 69,702