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Summary of Significant Accounting Policies - Reconciliation of Liabilities (Details) - Fair value, inputs, level 3 - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Transfer from equity   $ 29,413
Settlement of milestone #1 $ (1,446)  
Changes in fair value 846 (12,044)
Exercises   (2,311)
Transfer to equity   $ (15,058)
Ending balance 2,600  
SafeOp Surgical, Inc.    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Contingent consideration liability recorded upon acquisition of SafeOp $ 3,200