Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Carrying Value	$ 4,094.8	$ 3,481.2
Unfunded Commitments	220.9	93.4
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	27.5	56.2
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	262.4	250.3
Unfunded Commitments	214.1	93.4
Real estate equity investments
Schedule of Investments [Line Items]
Carrying Value	282.6
Unfunded Commitments	0.0
Other
Schedule of Investments [Line Items]
Carrying Value	4.9	4.5
Unfunded Commitments	6.8	0.0
Total other investments
Schedule of Investments [Line Items]
Carrying Value	$ 577.4	$ 311.0