CVIEs and CSIPs - Schedule of financial instruments of consolidated VIEs and consolidated SIPs not measured at fair value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 7,885.2		$ 7,596.0	$ 6,699.7	$ 6,323.1
Consolidated sponsored investment products [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	60.6		44.9
Debt of consolidated SIPs	105.7		122.3
Consolidated variable interest entities [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	66.6		74.3
Debt of consolidated variable interest entities	795.5		828.5
Carrying Value [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	7,758.0		7,476.8
Carrying Value [Member] | Consolidated sponsored investment products [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	60.6		44.9
Debt of consolidated SIPs	105.7		122.3
Carrying Value [Member] | Consolidated variable interest entities [Member]
Variable Interest Entity [Line Items]
Debt of consolidated variable interest entities	795.5
Estimated Fair Value [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	7,758.0		7,476.8
Estimated Fair Value [Member] | Consolidated sponsored investment products [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	60.6		44.9
Debt of consolidated SIPs	104.1		122.0
Estimated Fair Value [Member] | Consolidated variable interest entities [Member]
Variable Interest Entity [Line Items]
Debt of consolidated variable interest entities	$ 783.3	[1]
Cash and Cash Equivalents [Member]
Variable Interest Entity [Line Items]
Fair Value Level	1
Cash and Cash Equivalents [Member] | Consolidated sponsored investment products [Member]
Variable Interest Entity [Line Items]
Fair Value Level	1
Debt of consolidated SIPs [Member] | Consolidated sponsored investment products [Member]
Variable Interest Entity [Line Items]
Fair Value Level	3

[1] 1 Substantially all is Level 2.