Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Investments [Line Items]
Gross realized investment gains	$ 31.2	$ 7.7
Gross realized investment losses	(30.1)	(7.0)
Net realized investment gains on foreign currency exchange forward contracts	1.1	0.7
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	5.4	0.3
Gross realized investment losses	(1.7)	(2.1)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	0.8	1.1
Gross realized investment losses	(0.4)	(0.2)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	25.0	6.3
Gross realized investment losses	$ (28.0)	$ (4.7)