Borrowings - Outstanding Loans Payable of Consolidated Funds (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 16,945.5	$ 16,361.5
Fair Value	16,665.1	16,052.2
Senior secured notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	15,731.6	15,104.2
Fair Value	$ 15,460.0	$ 14,757.5
Weighted Average Interest Rate (as a percent)	1.86%	1.68%
Weighted Average Remaining Maturity in Years	9 years 3 months 25 days	9 years 2 months 16 days
Subordinated notes and preferred shares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 1,193.9	$ 1,242.3
Fair Value	$ 1,186.2	$ 1,278.8
Weighted Average Remaining Maturity in Years	8 years 6 months 7 days	8 years 3 months 11 days
Combination notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Borrowing Outstanding	$ 20.0	$ 15.0
Fair Value	$ 18.9	$ 15.9
Weighted Average Remaining Maturity in Years	78 months 4 days	7 years 1 month 21 days