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Debt - Impact of Accounting Changes (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt $ 1,178.7 $ 1,211.8
As previously reported    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   1,223.0
Accounting Standards Update 2015-03 | Adjustment    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   (11.2)
5% Convertible Senior Notes due 2017    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   331.5
5% Convertible Senior Notes due 2017 | As previously reported    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   333.5
5% Convertible Senior Notes due 2017 | Accounting Standards Update 2015-03 | Adjustment    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   (2.0)
2% Convertible Senior Notes due 2020    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   490.8
2% Convertible Senior Notes due 2020 | As previously reported    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   500.0
2% Convertible Senior Notes due 2020 | Accounting Standards Update 2015-03 | Adjustment    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   (9.2)
Convertible Junior Subordinated Debentures Due 2063    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   389.5
Convertible Junior Subordinated Debentures Due 2063 | As previously reported    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   389.5
Convertible Junior Subordinated Debentures Due 2063 | Accounting Standards Update 2015-03 | Adjustment    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Total long-term debt   $ 0.0