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Debt (Convertible Debentures) (Details)
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
Dec. 31, 2009
USD ($)
$ / shares
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument        
Deferred Tax Liabilities, Net, Noncurrent $ 209.1   $ 223.7 $ 222.6
Debt Instrument, Convertible, Threshold Trading Days 20      
Debt Instrument, Convertible, Threshold Consecutive Trading Days 30      
Temporary Equity, Par Value $ 58.4     $ 59.5
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger 130.00%      
Debt Instrument, Convertible, Stock Price Trigger | $ / shares $ 14.39      
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 $ 282.8 $ 56.3    
Deferred Tax Liabilities, Net, Noncurrent   $ 32.7    
Debt Instrument, Convertible, Terms of Conversion Feature 1000 1000    
Debt Instrument, Convertible, Conversion Price | $ / shares $ 11.07 $ 12.07    
Debt Instrument, Convertible, Conversion Ratio 90.37 82.82    
4.875% Unsecured Senior Notes [Member]        
Debt Instrument        
Debt Instrument, Interest Rate, Stated Percentage   4.875%    
Extinguishment of Debt, Amount   $ 122.5