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Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 283,644 $ 312,483 $ 172,128  
Restricted cash 6,541 5,652 3,265  
Total Cash, cash equivalents and restricted cash $ 290,185 $ 318,135 $ 175,393 $ 252,725