Fair Value Measurement - Changes in Financial Liabilities Using Level III Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Initial consolidation of funds	$ 13.2
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	565.9	$ 1,070.7
Included in other comprehensive income	(1,205.6)	(1,290.8)
Level III
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	16,266.7	15,534.7
Initial consolidation of funds	1,248.6	2,656.9
Borrowings	4,200.6	2,304.6
Paydowns	(3,557.2)	(3,472.2)
Issuances of equity		(1.8)
Sales	(7.4)	(4.4)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	(376.4)	12.3
Included in other comprehensive income	(602.9)	(763.4)
Balance, end of period	17,172.0	16,266.7
Changes in unrealized (gains) losses included in earnings related to financial liabilities still held at the reporting date	(467.2)	(80.0)
Level III | Loans Payable of Consolidated Funds
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	16,052.2	15,220.7
Initial consolidation of funds	1,248.6	2,656.9
Borrowings	4,196.7	2,251.2
Paydowns	(3,465.4)	(3,286.5)
Issuances of equity		0.0
Sales	0.0	0.0
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	(436.5)	(8.5)
Included in other comprehensive income	(548.9)	(781.6)
Balance, end of period	17,046.7	16,052.2
Changes in unrealized (gains) losses included in earnings related to financial liabilities still held at the reporting date	(489.1)	(101.8)
Level III | Derivative Instruments of Consolidated Funds
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	17.2	13.1
Initial consolidation of funds	0.0	0.0
Borrowings	0.0	0.0
Paydowns	0.0	0.0
Issuances of equity		0.0
Sales	(7.4)	(4.4)
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	20.9	10.6
Included in other comprehensive income	(1.6)	(2.1)
Balance, end of period	29.1	17.2
Changes in unrealized (gains) losses included in earnings related to financial liabilities still held at the reporting date	(16.5)	(7.4)
Level III | Contingent Consideration
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	51.1	178.8
Initial consolidation of funds	0.0	0.0
Borrowings	0.0	0.0
Paydowns	(22.6)	(97.9)
Issuances of equity		(1.8)
Sales	0.0	0.0
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	(7.4)	(27.4)
Included in other comprehensive income	(0.3)	(0.6)
Balance, end of period	20.8	51.1
Changes in unrealized (gains) losses included in earnings related to financial liabilities still held at the reporting date	(8.2)	(8.4)
Level III | Loans Payable of a consolidated real estate VIE
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	146.2	122.1
Initial consolidation of funds	0.0	0.0
Borrowings	3.9	53.4
Paydowns	(69.2)	(87.8)
Issuances of equity		0.0
Sales	0.0	0.0
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings [Abstract]
Included in earnings	46.6	37.6
Included in other comprehensive income	(52.1)	20.9
Balance, end of period	75.4	146.2
Changes in unrealized (gains) losses included in earnings related to financial liabilities still held at the reporting date	$ 46.6	$ 37.6