Variable Interest Entities and Consolidated Sponsored Investment Products - Narrative (Details) (USD $)	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2012
Number of sponsored investment products consolidated	30	36
Liquidation weighted average period	3 years 11 months 0 days	4 years 8 months 0 days
Consolidated SIPs' unfunded commitments	$ 139,200,000	$ 135,500,000
Unfunded commitments company contractually obligated to fund	3,100,000	2,800,000
Transfers into Level 2 from Level 1 - assets	0	0
Transfers into Level 1 from Level 2 - assets	0	0
Securities adjusted for quoted market prices			47,000,000
Number of countries impacted by price changes		16
Transfers out of Level 3 - assets	0	200,000
Transfers out of Level 3 - liabilities	0	0
Debt of consolidated variable interest entities	828,500,000	988,500,000
Investments in various funds held by consolidated SIPs for which fair value was estimated using NAV	695,800,000	442,000,000
AUM of non-consolidated VIEs	42,400,000,000	37,300,000,000
Collateralized Loan Obligations [Member]
Floating interest rate, minimum	0.46%	0.50%
Floating interest rate, maximum	9.73%	9.77%
Consolidated sponsored investment products [Member]
Interest Rate, Minimum	2.19%	2.45%
Interest Rate, Maximum	5.89%	5.83%
Transfers into Level 2 from Level 1 - assets	100,000	47,400,000
Transfers into Level 1 from Level 2 - assets	200,000	53,900,000
Transfers out of Level 3 - assets		0
Transfers out of Level 3 - liabilities		0
Level 3 [Member]
Transfers out of Level 3 - assets	1,300,000
Level 3 [Member] | Consolidated sponsored investment products [Member]
Investments in various funds held by consolidated SIPs for which fair value was estimated using NAV	393,500,000	243,600,000
Level 3 [Member] | Consolidated variable interest entities [Member]
Debt of consolidated variable interest entities	$ 47,200,000	$ 59,700,000