Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (5.4)	$ 2.6
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(5.4)	4.4
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(0.3)	(0.3)
Total return strategy [Member]
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ 0.3	$ (1.5)