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Summary of Significant Accounting Policies - Cash and Cash Equivalents (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]            
Cash and cash equivalents $ 113,524 $ 120,357 $ 138,285 $ 91,532 $ 99,565 $ 112,183
Cash equivalents 55,200   7,100      
Domestic Bank Accounts            
Cash and Cash Equivalents [Line Items]            
Cash and cash equivalents 79,300   83,100      
Foreign Bank Accounts            
Cash and Cash Equivalents [Line Items]            
Cash and cash equivalents 34,200   55,200      
Investments in Joint Ventures            
Cash and Cash Equivalents [Line Items]            
Cash and cash equivalents 12,400   16,700      
Domestic Joint Ventures            
Cash and Cash Equivalents [Line Items]            
Cash and cash equivalents $ 11,900   $ 10,000