Consolidated Investment Products - Schedule of Valuation Techniques and Significant Unobservable Inputs used in Level 3 Fair Value Measurements (Details) $ in Millions		Dec. 31, 2020 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		$ 546.2	$ 504.8
CIPs [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		3,228.9	2,709.2
Fair Value, Recurring [Member] | CIPs [Member] | Equity and debt securities
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		1,521.7	1,194.1
Fair Value, Recurring [Member] | CIPs [Member] | Real Estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		407.8	339.2
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		513.5	469.7
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		277.3	244.9
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Market pricing [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		124.7	116.3
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		$ 111.5	$ 108.5
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount rate [Member] | Minimum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.036	0.040
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount rate [Member] | Maximum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.186	0.230
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount rate [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.113	0.115
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount for lack of marketability [Member] | Minimum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.200	0.200
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount for lack of marketability [Member] | Maximum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.252	0.252
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount for lack of marketability [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.170	0.170
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Discount for lack of marketability [Member] | Weighted Average [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.219	0.219
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | EBITDA multiple [Member] | Minimum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		7.0	7.0
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | EBITDA multiple [Member] | Maximum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		19.1	19.1
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | EBITDA multiple [Member] | Weighted Average [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	11.0	10.8
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Risk premium [Member] | Minimum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.097	0.097
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Risk premium [Member] | Maximum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.193	0.193
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Risk premium [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.166	0.167
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Risk premium [Member] | Weighted Average [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.550	0.550
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Revenue multiple [Member] | Weighted Average [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	7.5	7.5
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Price-to-earnings ratio [Member] | Minimum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		9.4	9.4
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Price-to-earnings ratio [Member] | Maximum [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		10.0	10.0
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Price-to-earnings ratio [Member] | Weighted Average [Member] | Market comparable companies [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	9.7	9.7
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Private sale pricing [Member] | Minimum [Member] | Market pricing [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input | $ / shares		0.02	0.02
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Private sale pricing [Member] | Maximum [Member] | Market pricing [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input | $ / shares		100.00	100.00
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Equity and debt securities | Private sale pricing [Member] | Weighted Average [Member] | Market pricing [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input | $ / shares	[1]	11.66	13.01
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		$ 407.8	$ 339.2
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		296.0	231.8
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Yield capitalization [Member]
Schedule Of Consolidated Investment Products [Line Items]
Total investments, at fair value		$ 111.8	$ 107.4
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Discount rate [Member] | Minimum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.045	0.045
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Discount rate [Member] | Maximum [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.065	0.065
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Discount rate [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.051	0.052
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Equivalent yield [Member] | Minimum [Member] | Yield capitalization [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.043	0.043
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Equivalent yield [Member] | Maximum [Member] | Yield capitalization [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input		0.061	0.061
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Equivalent yield [Member] | Weighted Average [Member] | Yield capitalization [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.052	0.052
Fair Value, Recurring [Member] | CIPs [Member] | Level 3 [Member] | Real Estate [Member] | Exit Capitalization Rate | Weighted Average [Member] | Discounted cash flow [Member]
Schedule Of Consolidated Investment Products [Line Items]
Investments, measurement input	[1]	0.060	0.060

[1] Based on the relative fair value of the instruments.