Segment Reporting - Total Segments to Partnership Income before Provision for Taxes Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 679.1	$ 639.9	$ 2,275.5	$ 2,668.4
Expenses	615.6	492.6	1,848.6	2,007.5
Other income	(2.9)	18.6	12.0	76.4
Economic income	60.6	165.9	438.9	737.3
Total assets	13,433.7		13,433.7		$ 12,280.6
Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues	612.9	547.5	2,107.0	2,407.7
Expenses	502.3	344.8	1,555.3	1,504.8
Other income	0.0	0.0	0.0	0.0
Economic income	110.6	202.7	551.7	902.9
Total assets	8,213.0		8,213.0
Reconciling Items
Segment Reporting Information [Line Items]
Revenues	5.7	47.7	7.1	128.1
Expenses	62.3	95.5	136.0	305.5
Other income	0.0	0.0	0.0	0.0
Economic income	(56.6)	(47.8)	(128.9)	(177.4)
Total assets	(249.1)		(249.1)
Consolidated Funds | Consolidated Funds
Segment Reporting Information [Line Items]
Revenues	60.5	44.7	161.4	132.6
Expenses	51.0	52.3	157.3	197.2
Other income	(2.9)	18.6	12.0	76.4
Economic income	6.6	$ 11.0	16.1	$ 11.8
Total assets	$ 5,469.8		$ 5,469.8		$ 4,962.7