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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investments [Line Items]
Gross realized investment gains	$ 14.7	$ 4.2	$ 25.9	$ 9.8
Gross realized investment losses	(15.4)	(9.9)	(24.0)	(15.4)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(0.7)	(5.7)	1.9	(5.6)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	1.7	3.7	6.0	5.7
Gross realized investment losses	(3.7)	(3.7)	(4.3)	(5.9)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	8.7	0.0	13.0	0.5
Gross realized investment losses	(5.7)	(5.4)	(11.6)	(6.8)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	4.3	0.5	6.9	3.6
Gross realized investment losses	$ (6.0)	$ (0.8)	$ (8.1)	$ (2.7)