Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (1.7)	$ 0.9
Operational currency exposure
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(0.3)	0.4
Asset manager investment exposure
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	2.0	(0.2)
Total return strategy
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (3.4)	$ 0.7