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Summary of Significant Accounting Policies - Concentration of Credit Risk (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
bank
fund
Dec. 31, 2017
USD ($)
bank
fund
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Cash and Cash Equivalents [Line Items]        
Number of "Triple A" rated money market funds with cash, cash equivalent and restricted cash held on deposit | fund 0 12    
Number of global banks with cash, cash equivalent and restricted cash held on deposit | bank 7 7    
Money market funds $ 0 $ 585,000,000    
Maximum investment limit in any one financial institution 50,000,000      
Cash and cash equivalents $ 165,485,000 925,699,000 $ 236,484,000 $ 128,381,000
Time deposits   $ 24,482,000    
Investment in single mutual fund | Credit Concentration Risk        
Cash and Cash Equivalents [Line Items]        
Concentration risk percentage 1.00%