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Debt - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]        
Cash payments paid for interest     $ 58.6 $ 67.2
Amortization of treasury lock settlements $ 1.3 $ 1.4 4.0 4.2
Amortization of financing costs 0.7 $ 0.5 2.0 $ 1.5
6.50% Senior Notes due March 2018        
Debt Instrument [Line Items]        
Repayment of debt     150.0  
Fixed-Rate Senior Notes | Senior Notes        
Debt Instrument [Line Items]        
Book value of fixed rate debt 2,496.2   2,496.2  
Long-term debt (fixed-rate debt), fair value $ 2,478.0   $ 2,478.0