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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	$ 6.5	$ 3.5	$ 16.3	$ 29.4
Realized losses from foreign currency exchange forward contracts	(11.3)	(3.2)	(26.7)	(40.9)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(4.8)	0.3	(10.4)	(11.5)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	1.8	1.3	7.5	8.7
Realized losses from foreign currency exchange forward contracts	(4.2)	(0.8)	(10.1)	(16.3)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	0.6	0.0	1.1	2.3
Realized losses from foreign currency exchange forward contracts	(3.6)	(0.8)	(10.4)	(4.5)
Total return strategy [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	4.1	2.2	7.7	18.4
Realized losses from foreign currency exchange forward contracts	$ (3.5)	$ (1.6)	$ (6.2)	$ (20.1)