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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Gross realized investment gains	$ 42.2	$ 24.3	$ 45.5
Gross realized investment losses	(39.5)	(32.7)	(43.5)
Net realized investment gains (losses) on foreign currency exchange forward contracts	2.7	(8.4)	2.0
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	9.7	12.4	10.9
Gross realized investment losses	(7.9)	(13.8)	(18.0)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	5.8	1.5	9.0
Gross realized investment losses	(3.0)	(11.3)	(4.5)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	26.7	10.4	25.6
Gross realized investment losses	$ (28.6)	$ (7.6)	$ (21.0)