Consolidated Investment Products - Schedule of Balances of Assets and Liabilities of CIPs Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Assets [Abstract]
Investments, at fair value	$ 522.4	$ 551.6
CIPs [Member]
Assets [Abstract]
Investments, at fair value	2,040.2	2,109.4
Fair Value, Measurements, Recurring [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	522.4	551.6
Liability [Abstract]
NAV as a Practical Expedient	111.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	309.9	457.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	72.2	61.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	29.0	32.6
Fair Value, Measurements, Recurring [Member] | CIPs [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	2,040.2	2,109.4
Liability [Abstract]
Other liabilities		9.3
NAV as a Practical Expedient	137.5	113.8
Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	155.7	271.3
Liability [Abstract]
Other liabilities		0.6
Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	1,413.3	1,374.3
Liability [Abstract]
Other liabilities		8.7
Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	333.7	350.0
Liability [Abstract]
Other liabilities		0.0
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member]
Assets [Abstract]
Investments, at fair value	720.3	739.0
Liability [Abstract]
NAV as a Practical Expedient	137.5	113.8
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 1 [Member]
Assets [Abstract]
Investments, at fair value	155.4	270.7
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 2 [Member]
Assets [Abstract]
Investments, at fair value	233.8	154.8
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 3 [Member]
Assets [Abstract]
Investments, at fair value	193.6	199.7
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member]
Assets [Abstract]
Investments, at fair value	1,319.9	1,370.4
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 1 [Member]
Assets [Abstract]
Investments, at fair value	0.3	0.6
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 2 [Member]
Assets [Abstract]
Investments, at fair value	1,179.5	1,219.5
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | CIPs [Member] | Level 3 [Member]
Assets [Abstract]
Investments, at fair value	$ 140.1	$ 150.3