Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Other investments	$ 323.2	$ 387.2
Unfunded Commitments	108.9	64.2
Hedge funds
Schedule of Investments [Line Items]
Other investments	54.0	58.6
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Other investments	264.6	248.9
Unfunded Commitments	108.9	64.2
Overseas deposits
Schedule of Investments [Line Items]
Other investments	0.0	74.9
Unfunded Commitments	0.0	0.0
Other
Schedule of Investments [Line Items]
Other investments	4.6	4.8
Unfunded Commitments	$ 0.0	$ 0.0