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Summary of Significant Accounting Policies - Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Values of Financial Instruments    
Total carrying value of debt $ 1,914.5 $ 1,945.0
Senior Unsecured Notes    
Fair Values of Financial Instruments    
Carrying value, before deducting unamortized discount or premiums 750.0 750.0
Carrying value 744.0 743.2
Senior Unsecured Notes | Level 2    
Fair Values of Financial Instruments    
Fair value $ 831.7 $ 773.2