Segment Reporting - Total Segments to Partnership Income Before Provision for Taxes Reconciliation (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 460.3	$ 768.6	$ 1,061.1	$ 1,087.0	$ 151.7	$ 679.1	$ 893.6	$ 702.8	$ 3,377.0	$ 2,427.2	$ 3,676.2
Expenses	483.6	505.3	528.3	602.5	222.9	615.6	653.7	579.3	2,119.7	2,071.5	2,632.3
Other income	(17.0)	$ (1.9)	$ 9.2	$ (14.2)	(7.5)	$ (2.9)	$ 12.9	$ 2.0	(23.9)	4.5	88.4
Distributable earnings									1,233.4	360.2	1,132.3
Total assets	13,808.8				12,914.2				13,808.8	12,914.2	12,280.6
Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues									2,110.1	2,185.9	2,216.2
Expenses									1,463.5	1,512.0	1,546.2
Other income									0.0	0.0	0.0
Distributable earnings									646.6	673.9	670.0
Total assets	8,787.9				7,470.7				8,787.9	7,470.7
Reconciling Items
Segment Reporting Information [Line Items]
Revenues									1,067.7	26.8	1,282.3
Expenses									490.6	346.2	845.7
Other income									0.0	0.0	(35.1)
Distributable earnings									577.1	(319.4)	401.5
Total assets	(183.4)				(226.0)				(183.4)	(226.0)
Consolidated Funds | Consolidated Funds
Segment Reporting Information [Line Items]
Revenues									199.2	214.5	177.7
Expenses									165.6	213.3	240.4
Other income									(23.9)	4.5	123.5
Distributable earnings									9.7	5.7	$ 60.8
Total assets	$ 5,204.3				$ 5,669.5				$ 5,204.3	$ 5,669.5