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Long-term Debt - Convertible Senior Notes Due 2024 (Details)
1 Months Ended
Jul. 31, 2017
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]    
Principal amount   $ 338,750,000
2024 Convertible Notes    
Debt Instrument [Line Items]    
Principal amount $ 143,750,000.00  
Stated interest rate 2.875%  
Common Stock Class A Non-voting | 2024 Convertible Notes    
Debt Instrument [Line Items]    
Conversion rate per $1000 principal amount 100  
Conversion price, per share (in dollars per share) | $ / shares $ 10.00