Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 2.6	$ (0.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	4.4	(0.2)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(0.3)	(0.9)
Total return strategy [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (1.5)	$ 0.7