Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (2.3)	$ (0.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(2.2)	(0.2)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	0.2	(0.9)
Total return strategy [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (0.3)	$ 0.7