CVIEs and CSIPs - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Number of sponsored investment products consolidated	29		30
Gains from consolidated VIEs, net	$ 5,300,000	$ 5,700,000
Debt, unpaid principal balance	1,198,300,000		1,198,200,000
Liquidation weighted average period	3 years 7 months 0 days		3 years 10 months 24 days
Consolidated SIPs' unfunded commitments	128,000,000		139,200,000
Unfunded commitments company contractually obligated to fund	2,800,000		3,100,000
Transfers into Level 2 from Level 1 - assets	0	0
Transfers into Level 2 from Level 1 - liabilities	0	0
Transfers into Level 1 from Level 2 - assets	0	0
Transfers into Level 1 from Level 2 - liabilities	0	0
Investments in various funds held by consolidated SIPs for which fair value was estimated using NAV	436,800,000		695,800,000
AUM of non-consolidated VIEs	40,600,000,000		42,400,000,000
Consolidated sponsored investment products [Member]
Interest Rate, Minimum	2.19%		2.19%
Interest Rate, Maximum	5.81%		5.89%
Transfers into Level 2 from Level 1 - assets	0	0
Transfers into Level 2 from Level 1 - liabilities	0	0
Transfers into Level 1 from Level 2 - assets		100,000
Transfers into Level 1 from Level 2 - liabilities	0	0
Transfers into or out of Level 3 - assets	0	0
Transfers into or out of Level 3 - liabilities	0	0
Investments in various funds held by consolidated SIPs for which fair value was estimated using NAV	410,700,000		393,500,000
Consolidated variable interest entities [Member]
Floating interest rate, minimum	0.46%		0.46%
Floating interest rate, maximum	9.73%		9.73%
Debt of consolidated variable interest entities	795,500,000		828,500,000
Level 3 [Member] | Consolidated variable interest entities [Member]
Debt of consolidated variable interest entities			47,200,000
Collateralized Loan Obligations [Member]
Debt, unpaid principal balance	$ 825,500,000		$ 861,900,000