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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 29, 2016
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]        
Shares repurchased, weighted average price per share   $ 8.03    
Shares repurchased, value   $ 147.1    
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights)     1  
Common stock, beneficial ownership threshold to be considered an Acquiring Person (in hundredths)     5.00%  
Purchase price (in dollars per share)     $ 45  
Purchase price (in dollars per one-tenth share)     4.50  
Redemption price (in dollars per Right)     $ 0.001  
Rights        
Debt Instrument [Line Items]        
Period after person becomes acquiring person     10 days  
Common stock purchased per right     0.1  
5% Convertible Senior Notes due 2017        
Debt Instrument [Line Items]        
Extinguishment of debt, amount     $ 188.5 $ 11.5
Stated interest rate (in hundredths)     5.00%  
5% Convertible Senior Notes due 2017 | 2% Convertible Senior Notes        
Debt Instrument [Line Items]        
Extinguishment of debt, amount   $ 292.4    
Stated interest rate (in hundredths)     2.00%  
Repayments of debt (in shares)   18,300,000    
2% Convertible Senior Notes due 2020        
Debt Instrument [Line Items]        
Stated interest rate (in hundredths)     2.00%  
Convertible Junior Subordinated Debentures Due 2063        
Debt Instrument [Line Items]        
Stated interest rate (in hundredths)     9.00%  
9% Convertible Junior Subordinated Debentures due 2063        
Debt Instrument [Line Items]        
Extinguishment of debt, amount $ 132.7      
Stated interest rate (in hundredths)     9.00%  
Reacquisition of convertible junior subordinated debentures-equity component     $ 6.3