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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, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Carrying Value [1] $ 315.4 $ 312.5
Par value (in millions) [1] $ 430.8 $ 430.8
Debt instrument term 18 years  
[1]
The carrying value is accreted to the principal amount at maturity over a remaining life of 18 years.