Borrowings - Outstanding Loans Payable of Consolidated Funds (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 4,323.5	$ 3,876.6
Fair Value	4,303.8	3,866.3
Fair value of CLO	4,900.0	4,700.0
Senior secured notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	4,128.3	3,681.0
Fair Value	$ 4,100.5	$ 3,672.5
Weighted Average Interest Rate (as a percent)	2.16%	2.45%
Weighted Average Remaining Maturity in Years	11 years 5 months 8 days	10 years 2 months 19 days
Subordinated notes and preferred shares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 195.2	$ 195.6
Fair Value	$ 203.3	$ 193.8
Weighted Average Remaining Maturity in Years	9 years 10 months 6 days	9 years 3 months 3 days