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CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (unaudited) (Parenthetical) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Statement Of Cash Flows [Abstract]        
Cash and cash equivalents $ 244,673 $ 213,486    
Restricted cash and cash equivalents 2,635 2,635    
Total cash and cash equivalents and restricted cash and cash equivalents $ 247,308 $ 216,121 $ 162,999 $ 108,365