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Convertible Notes Payable - Narrative (Details)
1 Months Ended
Sep. 12, 2017
USD ($)
business_day
day
$ / shares
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Aug. 14, 2017
USD ($)
Debt Instrument [Line Items]        
Initial carrying amount   $ 187,434,000 $ 179,806,000  
Convertible debt | 3.00% Convertible notes        
Debt Instrument [Line Items]        
Aggregate principal amount $ 5,000,000.0     $ 300,000,000
Debt instrument stated rate 3.00%     3.00%
Debt issuance costs $ 9,400,000      
Debt instrument term 7 years      
Conversion ratio 0.0204891      
Initial conversion price (in dollars per share) | $ / shares $ 48.81      
Initial carrying amount   187,434,000   $ 166,300,000
Equity component   134,450,000   138,700,000
Proceeds from the issuance of debt   $ 305,000,000   $ 305,000,000.0
Debt discount, remaining discount amortization period 7 years      
Effective interest rate   13.04%    
Estimated fair value   $ 274,200,000    
Convertible debt | 3.00% Convertible notes | Stock price trigger at greater than or equal to 130%        
Debt Instrument [Line Items]        
Consecutive or non-consecutive trading days 20 days      
Consecutive trading days | day 30      
Percentage of stock price trigger 130.00%      
Convertible debt | 3.00% Convertible notes | Stock price trigger less than 98%        
Debt Instrument [Line Items]        
Consecutive trading days | day 5      
Percentage of stock price trigger 98.00%      
Trading days | business_day 5      
Convertible debt | 3.00% Convertible notes | Stock price trigger at greater than or equal to 130%, on or after September 1, 2021        
Debt Instrument [Line Items]        
Consecutive or non-consecutive trading days 20 days      
Consecutive trading days | day 30      
Percentage of stock price trigger 130.00%      
Consecutive trading days, redemption notice period | day 5      
Convertible debt | 3.00% Convertible notes | Additional Paid-In Capital        
Debt Instrument [Line Items]        
Debt issuance costs $ 4,300,000      
Convertible debt | 3.00% Convertible notes | Convertible notes        
Debt Instrument [Line Items]        
Debt issuance costs $ 5,100,000