Consolidated Investment Products - Schedule of Changes in Level 3 Assets of CIPs (Details) - CIPs [Member] - Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	$ 944.8	$ 646.7	$ 833.8	$ 597.1
Realized and unrealized gains included in investment and other income (losses) of consolidated investment products, net	3.9	(16.8)	24.5	(26.6)
Purchases	96.6	117.3	202.6	214.3
Sales and settlements	(92.2)	(4.5)	(126.8)	(8.6)
Transfers into Level 3	1.1	2.2	1.1	2.2
Transfers out of Level 3		1.0		1.0
Deconsolidation	(36.0)		(36.0)	(40.7)
Foreign exchange revaluation	(9.1)	(12.4)	9.9	(5.2)
Balance at End of Period - assets	909.1	731.5	909.1	731.5
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2021	4.1	(17.2)	24.0	(26.6)
Equity and debt securities
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	513.5	412.8	469.7	427.8
Realized and unrealized gains included in investment and other income (losses) of consolidated investment products, net	2.7	(17.3)	21.8	(24.3)
Purchases	75.4	40.0	130.3	72.7
Sales and settlements	(91.1)	(2.8)	(124.3)	(4.4)
Transfers into Level 3	1.1	2.2	1.1	2.2
Transfers out of Level 3		1.0		1.0
Deconsolidation	(36.0)		(36.0)	(40.7)
Foreign exchange revaluation	(2.4)	(1.4)	0.6	0.2
Balance at End of Period - assets	463.2	432.5	463.2	432.5
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2021	3.0	(17.3)	21.3	(24.3)
Loans [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	23.5	14.0	24.9	16.6
Realized and unrealized gains included in investment and other income (losses) of consolidated investment products, net	0.0	(1.3)	0.0	(1.4)
Purchases	0.0	0.0	0.0	0.0
Sales and settlements	(1.1)	(1.7)	0.0	(4.2)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3		0.0		0.0
Deconsolidation	0.0		0.0	0.0
Foreign exchange revaluation	0.0	0.0	0.0	0.0
Balance at End of Period - assets	22.4	11.0	22.4	11.0
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2021	(0.1)	(1.7)	2.7	(1.4)
Real Estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	407.8	219.9	339.2	152.7
Realized and unrealized gains included in investment and other income (losses) of consolidated investment products, net	1.2	1.8	2.7	(0.9)
Purchases	21.2	77.3	72.3	141.6
Sales and settlements	0.0	0.0	(2.5)	0.0
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3		0.0		0.0
Deconsolidation	0.0		0.0	0.0
Foreign exchange revaluation	(6.7)	(11.0)	9.3	(5.4)
Balance at End of Period - assets	423.5	288.0	423.5	288.0
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2021	$ 1.2	$ 1.8	$ 0.0	$ (0.9)

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Amount of sale and settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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[Line Items] for Tabular disclosure of qualitative and quantitative information related to investment products holds, whether or not such investment products (CIP) is included in the reporting entity's consolidated financial statements. Includes, but is not limited to, description of the significant judgments and assumptions made in determining whether a CIP (as defined) held by the entity requires the CIP (as defined) to be consolidated and (or) disclose information about its involvement with the CIP, individually or in aggregate (as applicable); the nature of restrictions, if any, on the CIP's assets and on the settlement of its liabilities reported by an entity in its statement of financial position, including the carrying amounts of such assets and liabilities; the nature of, and changes in, the risks associated with involvement in the CIP; how involvement with the CIP affects the entity's financial position, financial performance, and cash flows; the lack of recourse if creditors (or beneficial interest holders) of the CIP have no recourse to the general credit of the primary beneficiary (if applicable); the terms of arrangements, giving consideration to both explicit arrangements and implicit variable interests, if any, that could require the entity to provide financial support to the SIP, including events or circumstances that could expose the entity to a loss; the methodology used by the entity for determining whether or not it is the primary beneficiary of the consolidated investment products; the significant factors considered and judgments made in determining that the power to direct the activities of a CIP that most significantly impact the CIP's economic performance are shared (as defined); the carrying amounts and classification of assets and liabilities of the CIP included in the statement of financial position.

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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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