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Summary of Significant Accounting Policies - Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Values of Financial Instruments    
Total carrying value of debt $ 1,663.7 $ 1,596.3
Senior Unsecured Notes    
Fair Values of Financial Instruments    
Carrying value, before deducting unamortized discount or premiums 1,650.0 750.0
Carrying value 1,640.0 745.6
Senior Unsecured Notes | Level 2    
Fair Values of Financial Instruments    
Fair value $ 1,800.0 $ 840.0