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Long-Term Debt Long-Term Debt - Summary of Interest Expense (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]        
Amortization of debt discount and issuance costs(1)     $ 2,604 $ 1,489
Effective interest rate (as a percent) 5.45%   5.45%  
Senior Notes | Convertible Note Due 2023        
Debt Instrument [Line Items]        
Contractual interest expense $ 720 $ 0 $ 720 0
Amortization of debt discount and issuance costs(1) 938 0 938 0
Total $ 1,658 $ 0 $ 1,658 $ 0