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Summary of Significant Accounting Policies (Anti-Dilutive Shares Excluded from Computation of Diluted Net Loss per Share) (Details) - shares
shares in Thousands
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Stock Options [Member]    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Anti-dilutive shares outstanding excluded from computation 23,414 28,601
RSUs with Time-Based Vesting [Member]    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Anti-dilutive shares outstanding excluded from computation 1,121 355
RSUs with Performance-Based Vesting [Member]    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Anti-dilutive shares outstanding excluded from computation 138 652