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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Gross realized investment gains	$ 30.9	$ 42.2	$ 24.3
Gross realized investment losses	(29.0)	(39.5)	(32.7)
Net realized investment gains (losses) on foreign currency exchange forward contracts	1.9	2.7	(8.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	5.7	9.7	12.4
Gross realized investment losses	(10.6)	(7.9)	(13.8)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	2.7	5.8	1.5
Gross realized investment losses	(0.8)	(3.0)	(11.3)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	22.5	26.7	10.4
Gross realized investment losses	$ (17.6)	$ (28.6)	$ (7.6)