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Convertible Notes (Narrative) (Details)
12 Months Ended
Nov. 17, 2017
USD ($)
D
$ / shares
Aug. 16, 2013
USD ($)
D
$ / shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Nov. 14, 2017
$ / shares
shares
Debt instrument                      
Derivative, Nonmonetary Notional Amount, Shares | shares                     9,100,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares                     9,100,000
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares                     $ 23.30
Share Price | $ / shares                     $ 14.56
Potential Incremental Common Shares Attributable To Dilutive Effect Of Conversion Of Debt Securities | shares     9,100,000                
Additional paid in capital     $ 1,270,426,000     $ 1,280,051,000 $ 1,274,136,000 $ 1,264,000,000 $ 1,261,142,000 $ 1,254,344,000  
Payments for Hedge, Financing Activities         $ 33,500,000            
Proceeds from Issuance of Warrants         $ 23,200,000            
Convertible senior notes | 1.375% convertible notes due 2023                      
Debt instrument                      
Stock Price Premium                     60.00%
Face value $ 172,500,000   $ 172,500,000           $ 172,500,000    
Liability Component, Principal amount 137,300,000                    
Equity Component, Principal amount $ 35,200,000                    
Debt discount amortization period 5 years                    
Debt Issuance Cost, Convertible, Liability Component $ 2,600,000                    
Debt Issuance Cost, Convertible, Equity Component 700,000                    
Debt Issuance Costs, Gross $ 3,300,000                    
Conversion rate, number of shares to be issued per $1000 of principal (in shares) 52.8318                    
Principal amount of notes used as the denominator to determine number of shares converted into notes $ 1,000                    
Initial conversion price of notes (in dollars per share) | $ / shares $ 18.93                    
Debt Instrument, Convertible, Threshold Trading Days 20                    
Debt Instrument, Convertible, Threshold Consecutive Trading Days | D 30                    
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger 130.00%                    
Number of consecutive trading days before the five business days during the debt instrument measurement period 5 days                    
Denomination of the principal amount of notes used to calculate the percent of trading price during the debt instrument measurement period $ 1,000                    
Maximum conversion price as a percentage of closing stock price 98.00%                    
Percentage of face amount of debt instrument redeemable at the company's option 100.00%                    
Events of default                      
Period of default in payment of interest (in days) 30 days                    
Period of default to comply with other agreements (in days) 60 days                    
Minimum percentage of aggregate outstanding principal required for default event with other agreements 25.00%                    
Minimum principal amount of debt nonpayment required for debt default to occur $ 40,000,000.0                    
Period of nonpayment of principal amount required for debt default to occur (in days) 30 days                    
Minimum percentage of aggregate outstanding principal required for nonpayment of debt default to occur 25.00%                    
Minimum percentage of aggregate outstanding principal required for immediate payment declaration to occur 25.00%                    
Debt Instrument, Default Percent Of Principal And Accrued And Unpaid Interest 100.00%                    
Convertible senior notes | 1.125% Convertible Senior Notes due 2018                      
Debt instrument                      
Face value   $ 138,000,000.0                  
Liability Component, Principal amount   107,700,000                  
Equity Component, Principal amount   $ 30,300,000                  
Debt discount amortization period   5 years                  
Debt Issuance Cost, Convertible, Liability Component   $ 2,800,000                  
Debt Issuance Cost, Convertible, Equity Component   $ 800,000                  
Debt Instrument, Term   5 years                  
Debt Issuance Costs, Gross   $ 3,600,000                  
Conversion rate, number of shares to be issued per $1000 of principal (in shares)   82.8329                  
Principal amount of notes used as the denominator to determine number of shares converted into notes   $ 1,000                  
Initial conversion price of notes (in dollars per share) | $ / shares   $ 12.07                  
Debt Instrument, Convertible, Threshold Trading Days   20                  
Debt Instrument, Convertible, Threshold Consecutive Trading Days | D   30                  
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger   130.00%                  
Number of business days immediately after any ten consecutive trading day period during the note measurement period   5 days                  
Number of consecutive trading days before the five business days during the debt instrument measurement period   5 days                  
Denomination of the principal amount of notes used to calculate the percent of trading price during the debt instrument measurement period   $ 1,000                  
Maximum conversion price as a percentage of closing stock price   98.00%                  
Convertible senior notes                      
Debt instrument                      
Repayments of Debt       $ 81,200,000              
Issuance of common stock in connection with the maturity of the 2018 Notes related to the settlement of the in-the-money conversion feature of the 2018 Notes | shares       423,873              
Debt Instrument, Convertible, Beneficial Conversion Feature       $ 5,000,000.0              
Convertible senior notes | 1.375% convertible notes due 2023                      
Debt instrument                      
Stated Interest rate (as a percent) 1.375%   1.375% 1.375%         1.375%    
Convertible senior notes | 1.125% Convertible Senior Notes due 2018                      
Debt instrument                      
Stated Interest rate (as a percent)   1.125%   1.125%