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Debt (Convertible Debentures) (Details)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
$ / shares
Dec. 31, 2009
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Debt Instrument      
Deferred Tax Liabilities, Net, Noncurrent $ 202.1   $ 197.8
Debt Instrument, Convertible, Threshold Trading Days 20    
Debt Instrument, Convertible, Threshold Consecutive Trading Days 30 days    
Temporary Equity, Par Value $ 59.9   $ 61.3
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger 130.00%    
Debt Instrument, Convertible, Stock Price Trigger | $ / shares $ 14.57    
Maximum percentage of debenture trading price 98.00%    
5.75% Junior Subordinated Convertible Debentures [Member]      
Debt Instrument      
Debt Instrument, Face Amount   $ 125.0  
Debt Instrument, Interest Rate, Stated Percentage   5.75%  
Convertible Debt, Fair Value Disclosures $ 297.6 $ 56.3  
Deferred Tax Liabilities, Net, Noncurrent   $ 32.7  
Debt Instrument, Convertible, Terms of Conversion Feature 1000 1000  
Debt Instrument, Convertible, Conversion Price | $ / shares $ 11.21 $ 12.07  
Debt Instrument, Convertible, Conversion Ratio 89.22 82.82  
4.875% Unsecured Senior Notes [Member]      
Debt Instrument      
Debt Instrument, Interest Rate, Stated Percentage   4.875%  
Extinguishment of Debt, Amount   $ 122.5