Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Other investments	$ 311.0	$ 323.2
Unfunded Commitments	93.4	108.9
Hedge funds
Schedule of Investments [Line Items]
Other investments	56.2	54.0
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Other investments	250.3	264.6
Unfunded Commitments	93.4	108.9
Other
Schedule of Investments [Line Items]
Other investments	4.5	4.6
Unfunded Commitments	$ 0.0	$ 0.0