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Convertible Senior Notes and Capped Call Transactions - Narrative (Details)
$ / shares in Units, shares in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2020
USD ($)
day
$ / shares
shares
Mar. 31, 2019
USD ($)
equity_instrument
day
shares
Sep. 30, 2021
USD ($)
$ / shares
Sep. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
Sep. 30, 2021
USD ($)
$ / shares
Sep. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
$ / shares
Mar. 31, 2020
USD ($)
Sep. 30, 2019
USD ($)
$ / shares
Debt Instrument [Line Items]                    
Common stock, par value (in dollars per share) | $ / shares $ 0.001   $ 0.001     $ 0.001   $ 0.001   $ 0.001
Number of shares of common stock covered by called caps (shares) | shares 6,880 5,960                
Reduction to additional paid-in-capital related to called caps $ 46,100,000 $ 23,200,000                
Adjustments to additional paid in capital related to issuance costs attributable to equity component         $ 3,700,000          
2024 Notes                    
Debt Instrument [Line Items]                    
Carry amount of equity component of convertible notes                   $ 52,900,000
Total deferred issuance costs                   8,600,000
Debt issuance costs attributable to liability                   $ 6,600,000
Adjustments to additional paid in capital related to issuance costs attributable to equity component   2,000,000                
2026 Notes                    
Debt Instrument [Line Items]                    
Carry amount of equity component of convertible notes 162,500,000             $ 162,500,000    
Total deferred issuance costs 12,200,000             12,200,000    
Debt issuance costs attributable to liability $ 8,500,000             $ 8,500,000    
Capped calls                    
Debt Instrument [Line Items]                    
Capped caps initial strike price (in dollars per share) | $ / shares $ 75.23             $ 75.23   $ 38.58
Capped caps initial cap price (in dollars per share) | $ / shares $ 105.58             $ 105.58   $ 57.16
Convertible Debt | 2024 Notes                    
Debt Instrument [Line Items]                    
Aggregate principal                 $ 230,000,000  
Debt instrument stated rate (percent)                 0.75%  
Aggregate principal from exercise in full of over-allotment options                 $ 30,000,000  
Proceeds from debt offering, net of debt issuance costs   $ 221,400,000                
Debt instrument, unit of principal for conversion                   $ 1,000
Number of shares per convertible note | equity_instrument   25.9182                
Convertible debt conversion price (in dollars per share) | $ / shares                   $ 38.58
Percentage of principal amount paid if repurchase due to fundamental change (percent)   100.00%                
Threshold trading days in consideration of note conversion | day   20                
Threshold consecutive trading days in analysis of conversion price | day   30                
Threshold percentage of stock price if converted   130.00%                
Threshold for five day period, product of sale price of common stock and conversion rate of notes   98.00%                
Remaining amortization period for debt discount and debt issuance costs           2 years 4 months 24 days        
Effective interest rate (percent)           0.0374%        
Convertible Debt | 2026 Notes                    
Debt Instrument [Line Items]                    
Aggregate principal $ 517,500,000             $ 517,500,000    
Debt instrument stated rate (percent) 0.00%             0.00%    
Aggregate principal from exercise in full of over-allotment options $ 67,500,000             $ 67,500,000    
Proceeds from debt offering, net of debt issuance costs               505,300,000    
Debt instrument, unit of principal for conversion $ 1,000             $ 1,000    
Number of shares per convertible note 13.2933                  
Convertible debt conversion price (in dollars per share) | $ / shares $ 75.23             $ 75.23    
Percentage of principal amount paid if repurchase due to fundamental change (percent) 100.00%                  
Threshold trading days in consideration of note conversion | day 20                  
Threshold consecutive trading days in analysis of conversion price | day 30                  
Threshold percentage of stock price if converted 130.00%                  
Threshold for five day period, product of sale price of common stock and conversion rate of notes 98.00%                  
Remaining amortization period for debt discount and debt issuance costs           5 years 2 months 12 days        
Effective interest rate (percent)           0.0577%        
Convertible Debt | Convertible Senior Notes                    
Debt Instrument [Line Items]                    
Carry amount of equity component of convertible notes     $ 215,434,000     $ 215,434,000        
Interest expense     $ 9,457,000 $ 3,189,000   $ 27,922,000 $ 9,429,000