Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Schedule of Investments [Line Items]
Gross realized investment gains	$ 2.6	$ 4.1
Gross realized investment losses	(2.7)	(3.4)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(0.1)	0.7
Operational currency
Schedule of Investments [Line Items]
Gross realized investment gains	1.4	4.1
Gross realized investment losses	(2.7)	(3.1)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	1.2	0.0
Gross realized investment losses	$ 0.0	$ (0.3)