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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Investments [Line Items]
Gross realized investment gains	$ 9.6	$ 7.4	$ 24.1	$ 33.3
Gross realized investment losses	(10.4)	(8.8)	(24.1)	(32.8)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(0.8)	(1.4)	0.0	0.5
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	1.3	0.4	2.2	6.4
Gross realized investment losses	(3.2)	(2.7)	(7.8)	(7.0)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	1.8	1.4	3.1	14.4
Gross realized investment losses	(0.1)	(0.9)	(0.6)	(12.5)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	6.5	5.6	18.8	12.5
Gross realized investment losses	$ (7.1)	$ (5.2)	$ (15.7)	$ (13.3)