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Convertible Notes (Narrative) (Details)
3 Months Ended 12 Months Ended
Nov. 17, 2017
USD ($)
D
$ / shares
Aug. 16, 2013
USD ($)
D
$ / shares
Sep. 30, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Nov. 14, 2017
$ / shares
Debt Instrument [Line Items]                
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
Proceeds from Issuance of Warrants         $ 0 $ 23,173,000 $ 0  
Potential Incremental Common Shares Attributable To Dilutive Effect Of Conversion Of Debt Securities | shares       9,100,000        
Additional paid in capital       $ 1,212,798,000 1,226,588,000 1,212,798,000    
Events of default                
Payments for Hedge, Financing Activities         0 33,523,000 $ 0  
Convertible notes                
Debt Instrument [Line Items]                
Additional paid in capital       111,300,000 111,300,000 111,300,000    
Convertible notes | Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]                
Debt Instrument [Line Items]                
Stock Price Premium               60.00%
Face Value $ 172,500,000     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       4 years 1 month 2 days      
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 notes | 1.125% Convertible Senior Notes due 2018                
Debt Instrument [Line Items]                
Face Value   $ 138,000,000   81,207,000 $ 0 81,207,000    
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%            
Percentage of face amount of debt instrument redeemable at the company's option   100.00%            
Debt Instrument, Repurchase Amount       $ 56,800,000   $ 56,800,000    
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            
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%            
Convertible Senior Notes [Member]                
Events of default                
Repayments of Debt     $ 81,200,000          
Debt Conversion, Converted Instrument, Shares Issued | shares     423,873          
Debt Instrument, Convertible, Beneficial Conversion Feature     $ 5,000,000          
Convertible Senior Notes [Member] | Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]                
Debt Instrument [Line Items]                
Stated Interest rate (as a percent) 1.375%       1.375%      
Convertible Senior Notes [Member] | 1.125% Convertible Senior Notes due 2018                
Debt Instrument [Line Items]                
Stated Interest rate (as a percent)   1.125%   1.125% 1.125% 1.125%