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Summary of Significant Accounting Policies - Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Values of Financial Instruments    
Total carrying value of debt $ 2,214.2 $ 1,914.5
Senior Unsecured Notes    
Fair Values of Financial Instruments    
Carrying value, before deducting unamortized discount or premiums 750.0 750.0
Carrying value 744.8 744.0
Senior Unsecured Notes | Level 2    
Fair Values of Financial Instruments    
Fair value $ 780.0 $ 831.7