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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investments [Line Items]
Gross realized investment gains	$ 7.4	$ 6.5	$ 33.3	$ 16.3
Gross realized investment losses	(8.8)	(11.3)	(32.8)	(26.7)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(1.4)	(4.8)	0.5	(10.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	0.4	1.8	6.4	7.5
Gross realized investment losses	(2.7)	(4.2)	(7.0)	(10.1)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	1.4	0.6	14.4	1.1
Gross realized investment losses	(0.9)	(3.6)	(12.5)	(10.4)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	5.6	4.1	12.5	7.7
Gross realized investment losses	$ (5.2)	$ (3.5)	$ (13.3)	$ (6.2)