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Summary of Significant Accounting Policies - Anti-dilutive Securities (Details) - shares
shares in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock options to purchase common stock      
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding      
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) 9,382 8,609 7,117
Unvested restricted stock units      
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding      
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) 228 47 89
Convertible debt securities      
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding      
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) 11,492 0 0