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Debt - Carrying Value and Principal Amount at Maturity of Junior Convertible Securities (Details) - Junior convertible securities - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Carrying Value $ 318.4 $ 315.4
Principal Amount at Maturity $ 430.8 $ 430.8
Debt instrument term 17 years