Debt - Narrative (Details)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 20, 2014
Sep. 30, 2016
USD ($)
$ / shares
Jul. 31, 2014
USD ($)
shares
Jun. 30, 2014
USD ($)
$ / shares
Jun. 30, 2011
USD ($)
$ / shares
Sep. 30, 2016
USD ($)
$ / shares
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
$ / shares
Sep. 30, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]                      
Unamortized discount   $ 73,148,000       $ 73,148,000   $ 73,148,000     $ 25,704,000
Shares Issued with debt conversion (shares) | shares     348,535                
Issuance of common stock pursuant to conversion of debt     $ 12,600,000                
Loss on extinguishment of long-term debt           $ (2,551,000) $ 0 (2,551,000) $ 0 $ (23,200,000)  
Non-cash interest               $ 6,117,000 5,721,000    
Investor                      
Debt Instrument [Line Items]                      
Principal amount of modified debt held         $ 73,000,000.0            
2% Convertible Senior Notes                      
Debt Instrument [Line Items]                      
Debt, interest rate   2.00%   2.00%   2.00%   2.00%      
Principal amount of Senior Notes       $ 201,300,000.0              
Debt conversion rate       21.5019              
Principal amount per note used in conversion rate       $ 1,000              
Conversion price, per share (USD per share) | $ / shares       $ 46.51              
Nonconvertible debt borrowing rate       6.20%              
Deferred financing costs, amortization period       5 years              
Debt discount amortization period       5 years              
Finance costs reclassified against equity       $ 1,200,000              
Portion of 2% Notes purchased by 3.75% holders       98,200,000              
Amount allocated to debt   $ 121,400,000                  
Amount allocated to equity   32,900,000                  
Long-term debt   59,100,000       $ 59,100,000   $ 59,100,000     $ 171,700,000
Loss on extinguishment of long-term debt           (2,600,000)   (2,600,000)      
Deferred financing costs, gross       $ 6,700,000              
Interest expense related to Notes           900,000 1,000,000 2,900,000 3,000,000    
Non-cash interest           1,600,000 $ 1,900,000 5,600,000 $ 5,700,000    
2% Convertible Senior Notes | First 180 days                      
Debt Instrument [Line Items]                      
Debt instrument, additional interest in event of reporting violation       0.50%              
Debt instrument, redemption period       360 days              
2% Convertible Senior Notes | Up to 360 days                      
Debt Instrument [Line Items]                      
Debt instrument, additional interest in event of reporting violation       0.25%              
Debt instrument, redemption period       180 days              
2% Convertible Senior Notes | Investor                      
Debt Instrument [Line Items]                      
Unamortized discount       $ 35,600,000              
2% Convertible Senior Notes | Investor                      
Debt Instrument [Line Items]                      
Principal amount of Senior Notes   134,200,000.0       134,200,000.0   134,200,000.0      
Payments of long-term debt   153,600,000                  
Interest payable   $ 700,000       $ 700,000   $ 700,000      
1.25% Convertible Senior Notes                      
Debt Instrument [Line Items]                      
Debt, interest rate   1.25%       1.25%   1.25%      
Principal amount of Senior Notes   $ 345,000,000       $ 345,000,000   $ 345,000,000      
Debt conversion rate   17.1332                  
Principal amount per note used in conversion rate   $ 1,000       $ 1,000   $ 1,000      
Conversion price, per share (USD per share) | $ / shares   $ 58.37       $ 58.37   $ 58.37      
Nonconvertible debt borrowing rate   5.80%       5.80%   5.80%      
Deferred financing costs, amortization period   5 years                  
Debt discount amortization period   5 years                  
Finance costs reclassified against equity   $ 2,100,000                  
Long-term debt   269,900,000       $ 269,900,000   $ 269,900,000      
Deferred financing costs, gross   $ 11,100,000       11,100,000   11,100,000      
Debt instrument, additional interest in event of reporting violation   0.50%                  
Interest expense related to Notes           200,000   200,000      
Non-cash interest           500,000   500,000      
1.25% Convertible Senior Notes | Investor                      
Debt Instrument [Line Items]                      
Unamortized discount   $ 66,700,000       $ 66,700,000   $ 66,700,000      
3.75% Convertible Notes                      
Debt Instrument [Line Items]                      
Debt, interest rate         3.75%            
Principal amount of Senior Notes         $ 143,800,000.0            
Debt conversion rate         38.1749            
Principal amount per note used in conversion rate         $ 1,000            
Conversion price, per share (USD per share) | $ / shares         $ 26.20            
Payments of long-term debt       160,700,000              
Deferred financing costs, amortization period         5 years            
Debt instrument, repurchased face amount       114,900,000              
Debt Instrument repurchase premium in dollars       45,800,000              
Principal amount of old debt held       80,000,000              
Amount allocated to debt     27,900,000 112,400,000           140,300,000  
Amount allocated to equity     13,500,000 48,300,000              
Percentage required of the last reported sale price per share of the Company's common stock for redemption 130.00%                    
Number of trading days 20 days                    
Number of consecutive trading days 30 days                    
Long-term debt       $ 28,800,000           $ 0  
Principal amount of debt converted     28,500,000                
Total consideration paid     41,100,000                
Debt instrument, redemption, principal amount     300,000                
Repayment of remaining principal amount including interest     $ 300,000                
3.75% Convertible Notes | Modified Debt                      
Debt Instrument [Line Items]                      
Unamortized discount         $ 25,800,000            
Nonconvertible debt borrowing rate         16.50%            
Transaction fees         $ 2,000,000            
3.75% Convertible Notes | Debt discount related to premium payment in connection with the purchase of                      
Debt Instrument [Line Items]                      
Unamortized discount         15,100,000            
3.75% Convertible Notes | Debt discount related to the increase in the value of the conversion feature.                      
Debt Instrument [Line Items]                      
Unamortized discount         200,000            
3.75% Convertible Notes | New Debt                      
Debt Instrument [Line Items]                      
Unamortized discount         $ 26,600,000            
Nonconvertible debt borrowing rate         12.40%            
Principal debt amount issued to new investors         $ 84,300,000            
Debt discount amortization period         5 years            
Deferred financing costs         $ 2,800,000            
Finance costs reclassified against equity         900,000            
3.75% Convertible Notes | Investor                      
Debt Instrument [Line Items]                      
Principal amount of modified debt held         $ 59,500,000.0            
5.375% Convertible Notes                      
Debt Instrument [Line Items]                      
Debt, interest rate         5.375%            
Payments of long-term debt         $ 85,100,000            
5.375% Convertible Notes | Investor                      
Debt Instrument [Line Items]                      
Repurchase premium         21.50%            
Unamortized discount         $ 10,500,000            
5.375% Convertible Notes | Investor                      
Debt Instrument [Line Items]                      
Principal amount of Senior Notes         70,000,000            
Principal amount of modified debt held         $ 13,500,000