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Debt - Interest Expense - Convertible Senior Notes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Non-Cash Interest Expense      
Total Interest Expense $ 19,457 $ 222 $ 464
Convertible Senior Notes      
Cash Interest Expense      
Coupon interest expense 8,901    
Non-Cash Interest Expense      
Amortization of debt discount 9,490    
Amortization of transaction costs 956    
Total Interest Expense 19,347    
Convertible Senior Notes | Level 2      
Non-Cash Interest Expense      
Fair value $ 300,700