Accrued Performance Fees - Components of Accrued Giveback Obligations (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Accrued giveback obligations	$ (66.8)	$ (160.8)
Corporate Private Equity
Segment Reporting Information [Line Items]
Accrued giveback obligations	(8.7)	(3.9)
Real Assets
Segment Reporting Information [Line Items]
Accrued giveback obligations	$ (58.1)	$ (156.9)