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Debt - Convertible Notes (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Sep. 04, 2019
$ / shares
$ / unit
Sep. 30, 2019
USD ($)
d
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
$ / shares
Debt Instrument [Line Items]              
Debt instrument trading days | d   20          
Debt instrument consecutive trading day | d   30          
Premiums for capped call options   $ 32,900,000     $ 32,890,000    
Call Option              
Debt Instrument [Line Items]              
Capped price (in dollars per share) | $ / unit 37.00            
Premium of cap price as a percentage of closing price 100.00%            
Convertible debt | Convertible Senior Notes Due 2024              
Debt Instrument [Line Items]              
Principal amount   $ 316,250,000 $ 316,250,000 $ 316,250,000 316,250,000 $ 316,250,000  
Debt instrument, interest rate percentage   1.50%   1.50%      
Debt instrument, effective conversion price (in dollars per share) | $ / shares $ 24.98            
Stock price trigger, premium on closing price (as a percent) 35.00%            
Redemption price (as a percent) 100.00% 100.00%          
Stock price trigger (as a percent)   130.00%          
Debt issuance costs incurred     9,200,000        
Unamortized issuance costs   $ 5,500,000 $ 5,197,000 $ 5,500,000 $ 5,197,000 $ 4,312,000  
Debt issuance costs related with equity component of convertible debt       3,700,000      
Equity component of convertible debt   128,400,000   128,400,000     $ 128,400,000
Convertible debt | Convertible Senior Notes Due 2024 | LEVEL 3              
Debt Instrument [Line Items]              
Convertible notes, fair value of liability component   $ 187,900,000   $ 187,900,000      
Convertible notes fair value of liability component, effective interest rate percentage           10.50%  
Common Stock              
Debt Instrument [Line Items]              
Closing stock price (in dollars per share) | $ / shares $ 18.50            
Common Stock | Convertible debt | Convertible Senior Notes Due 2024              
Debt Instrument [Line Items]              
Debt instrument, conversion price (in dollars per share) | $ / shares             $ 0.04004