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Convertible Senior Notes, Net - Summary of Note Terms (Details) - Convertible Debt
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]        
Principal Amount   $ 575,000,000    
Conversion rate basis, principal amount   $ 1,000,000    
2022 Notes        
Debt Instrument [Line Items]        
Principal Amount       $ 345,000,000
Coupon Rate       4.75%
Effective Rate       5.60%
Conversion Rate       0.0502260
Remaining Period of Amortization       2 years 10 months 24 days
2023 Notes        
Debt Instrument [Line Items]        
Principal Amount $ 230,000,000   $ 230,000,000  
Coupon Rate 5.375%   5.375%  
Effective Rate 6.16%   6.16%  
Conversion Rate 0.0487187      
Remaining Period of Amortization     4 years 14 days