Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Carrying Value	$ 3,976.0	$ 3,481.2
Unfunded Commitments	112.6	93.4
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	28.7	56.2
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	258.4	250.3
Unfunded Commitments	82.2	93.4
Real estate equity investments
Schedule of Investments [Line Items]
Carrying Value	101.0	0.0
Unfunded Commitments	0.0	0.0
Other
Schedule of Investments [Line Items]
Carrying Value	42.2	4.5
Unfunded Commitments	30.4	0.0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	$ 430.3	$ 311.0