Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Carrying Value	$ 4,011.1	$ 3,481.2
Unfunded Commitments	93.7	93.4
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	31.5	56.2
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	257.9	250.3
Unfunded Commitments	82.4	93.4
Other
Schedule of Investments [Line Items]
Carrying Value	55.0	4.5
Unfunded Commitments	11.3	0.0
Other Invested Assets
Schedule of Investments [Line Items]
Carrying Value	$ 344.4	$ 311.0