Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Schedule Of Investments [Line Items]
Fair value of foreign currency exchange forward contracts		$ 4.0	$ 7.3
Operational Currency Exposure [Member]
Schedule Of Investments [Line Items]
Fair value of foreign currency exchange forward contracts	[1]	0.0	5.2
Asset Manager Investment Exposure [Member]
Schedule Of Investments [Line Items]
Fair value of foreign currency exchange forward contracts		0.7	2.9
Total return strategy [Member]
Schedule Of Investments [Line Items]
Fair value of foreign currency exchange forward contracts		$ 3.3	$ (0.8)

[1] CAD currency exposure less than $0.1 million.