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Debt - Convertible Notes (Details) - 2015 Convertible Notes
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
May 31, 2015
USD ($)
D
$ / shares
Dec. 31, 2018
Convertible Notes    
Amount of debt issued | $ $ 25,000  
Interest rate of debt (as a percent) 7.50%  
Transaction costs | $ $ 50  
Conversion price (in dollars per share) | $ / shares $ 21.50  
Conditional conversion price (in dollars per share) | $ / shares $ 30.10  
Number of trading days for which stock price must exceed specified price | D 20  
Number of consecutive trading days during which stock price must exceed specified price | D 30  
Convertible notes voting restriction period   3 years