Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ 0.3	$ 1.3
Operational currency
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(0.7)	1.5
Open contracts in a gain position
Schedule of Investments [Line Items]
Foreign currency exchange, notional amount	0.0	75.4
Foreign currency exchange, fair value	0.0	1.8
Open contracts in a loss position
Schedule of Investments [Line Items]
Foreign currency exchange, notional amount	100.4	43.3
Foreign currency exchange, fair value	(0.7)	(0.3)
Asset manager investment exposure
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	1.0	(0.2)
Open contracts in a gain position
Schedule of Investments [Line Items]
Foreign currency exchange, notional amount	45.8	0.0
Foreign currency exchange, fair value	1.0	0.0
Open contracts in a loss position
Schedule of Investments [Line Items]
Foreign currency exchange, notional amount	0.0	45.8
Foreign currency exchange, fair value	$ 0.0	$ (0.2)