Borrowings - Outstanding Loans Payable of Consolidated Funds (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 2,398.0	$ 1,981.1
Loans Payable of Consolidated Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	6,010.3	5,606.4
Fair Value	5,938.3	5,563.0
Senior secured notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	5,697.8	5,442.2
Fair Value	$ 5,700.1	$ 5,358.9
Weighted Average Interest Rate	1.72%	1.74%
Weighted Average Remaining Maturity in Years	10 years 5 months 15 days	10 years 4 months 9 days
Subordinated notes, preferred shares and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 312.5	$ 164.2
Fair Value	$ 238.2	$ 204.1
Weighted Average Remaining Maturity in Years	10 years 8 months 1 day	10 years 5 months 26 days