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Acquisition of Tokio Millennium Re (Defined Benefit Pension Plan) (Details) - Pension Plan - TMR - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Business Acquisition [Line Items]    
Net balance sheet liability $ 6.4 $ 6.5
Projected benefit obligation 20.1 20.4
Plan assets at fair value $ 13.6 $ 13.9