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Description of Business and Basis of Presentation Description of Business and Basis of Presentation - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]                        
Cash and cash equivalents $ 495,072       $ 381,165       $ 495,072 $ 381,165 $ 162,591 $ 282,876
Net loss $ 91,573 $ 87,743 $ 76,437 $ 68,524 $ 65,384 $ 45,179 $ 44,672 $ 37,414 $ 324,277 $ 192,649 $ 176,273