Summary of Significant Accounting Policies - Principles of Consolidation (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2016	Dec. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Deconsolidated assets	$ (12,280.6)	$ (9,973.0)		$ (32,181.6)
Deconsolidated liabilities	(9,331.6)	(8,519.0)
Adjustment to partners’ capital	2,949.0	$ 1,454.0		$ 6,077.6	$ 9,094.5
Consolidated VIEs, assets	5,000.0
Consolidated VIEs, liabilities	4,800.0
Collateralized Loan Obligations
Variable Interest Entity [Line Items]
Investment in CLOs	$ 220.6
Accounting Standards Update 2015-2
Variable Interest Entity [Line Items]
Adjustment to redeemable noncontrolling interest			$ 2,800.0
Accounting Standards Update 2015-2 | Restatement Adjustment
Variable Interest Entity [Line Items]
Deconsolidated assets			23,200.0
Deconsolidated liabilities			16,100.0
Adjustment to partners’ capital			$ 4,300.0