Fair Value Measurement - Partnership's Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value On Recurring Basis [Line Items]
Total Assets	$ 26,090.5	$ 24,836.3
Loans payable of Consolidated Funds	15,019.8	13,656.7
Interest rate swaps	7.8	10.5
Derivative instruments of the CLOs	8.9	15.8
Contingent consideration	229.1	244.3
Total Liabilities	15,265.6	13,927.3
Equity Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,424.9	3,379.9
Bonds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	1,058.9	934.2
Loans [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	13,771.2	13,290.1
Partnerships and LLC Interests [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,840.6	4,315.5
Hedge Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,961.0	2,888.7
Other [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	11.5	7.3
Investments of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	26,068.1	24,815.7
Trading Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	10.0	20.0
Restricted Securities of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	12.4	0.6
Level I [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	801.1	873.4
Level I [Member] | Equity Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	788.7	872.8
Level I [Member] | Investments of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	788.7	872.8
Level I [Member] | Restricted Securities of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	12.4	0.6
Level II [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,979.6	2,920.7
Interest rate swaps	7.8	10.5
Contingent consideration	56.9	57.6
Total Liabilities	64.7	68.1
Level II [Member] | Equity Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	18.6	32.0
Level II [Member] | Hedge Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,961.0	2,888.7
Level II [Member] | Investments of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,979.6	2,920.7
Level III [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	21,309.8	21,042.2
Loans payable of Consolidated Funds	15,019.8	13,656.7
Derivative instruments of the CLOs	8.9	15.8
Contingent consideration	172.2	186.7
Total Liabilities	15,200.9	13,859.2
Level III [Member] | Equity Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	2,617.6	2,475.1
Level III [Member] | Bonds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	1,058.9	934.2
Level III [Member] | Loans [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	13,771.2	13,290.1
Level III [Member] | Partnerships and LLC Interests [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	3,840.6	4,315.5
Level III [Member] | Other [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	11.5	7.3
Level III [Member] | Investments of Consolidated Funds [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	21,299.8	21,022.2
Level III [Member] | Trading Securities [Member]
Fair Value On Recurring Basis [Line Items]
Total Assets	$ 10.0	$ 20.0