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Debt - Convertible Notes (Details)
3 Months Ended 9 Months Ended
Feb. 15, 2019
USD ($)
Jun. 30, 2017
USD ($)
day
$ / shares
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Feb. 28, 2014
USD ($)
Debt Instrument [Line Items]            
Extinguishment of debt     $ 21,417,000 $ 0    
Convertible Notes due 2022 | Convertible Debt            
Debt Instrument [Line Items]            
Period of amortization of debt discount to interest expense using effective interest method   5 years 5 years      
Aggregate principal amount of debt issued   $ 320,000,000.0        
Proceeds from 2022 Convertible Notes   310,400,000        
Payment of convertible debt issuance costs   9,600,000        
Debt issuance costs   9,600,000        
Debt issuance costs, allocated to equity component   2,200,000        
Debt issuance costs allocated to the liability component   $ 7,400,000        
Interest rate   3.25%        
Debt conversion ratio   0.0365464        
Initial conversion price of convertible notes into common stock (in usd per share) | $ / shares   $ 27.36        
Debt term   5 years        
Effective interest rate on liability component     9.49%      
Convertible Notes due 2019 | Convertible Debt            
Debt Instrument [Line Items]            
Aggregate principal amount of debt issued           $ 200,000,000.0
Repurchase amount         $ 178,500,000  
Repurchase price         $ 192,700,000  
Extinguishment of debt $ 21,400,000          
Debt Instrument, Conversion, Period One | Convertible Notes due 2022 | Convertible Debt            
Debt Instrument [Line Items]            
Trading period | day   20        
Consecutive trading period | day   30        
Closing sales price of the entity's common stock that the conversion price must exceed or be equal in order for the notes to be convertible   130.00%        
Debt Instrument, Conversion, Period Two | Convertible Notes due 2022 | Convertible Debt            
Debt Instrument [Line Items]            
Consecutive trading period | day   5        
Closing sales price of the entity's common stock that the conversion price must exceed or be equal in order for the notes to be convertible   98.00%        
Consecutive business days after any five consecutive trading day period during the note measurement period | day   5