Consolidated Investment Products - Schedule of Changes in Level 3 Assets of CIPs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Schedule Of Consolidated Investment Products [Line Items]
Transfers into Level 3	$ 0.0	$ 0.0	$ 0.0	$ 0.0
CIPs [Member]
Schedule Of Consolidated Investment Products [Line Items]
Transfers into Level 3	(2.2)	0.0	(2.2)	(0.1)
Transfers out of Level 3	(1.0)	0.0	(1.0)	(29.0)
CIPs [Member] \| Level 3 [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	646.7	333.7	597.1	350.0
Acquisition		84.9		84.9
Realized and unrealized gains (losses) included in investment and other income (losses), net	(16.8)	(5.4)	(26.6)	(2.2)
Purchases	117.3	60.7	214.3	78.6
Sales and settlements	(4.5)		(8.6)	(22.2)
Sales		(14.8)
Deconsolidation			(40.7)
Transfers into Level 3	2.2		2.2	(0.1)
Transfers out of Level 3	(1.0)		(1.0)	(29.0)
Foreign exchange revaluation	(12.4)	(2.3)	(5.2)	(3.4)
Balance at End of Period - assets	731.5	456.8	731.5	456.8
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2020	(17.2)	(4.8)	(26.6)	5.6
CIPs [Member] \| Level 3 [Member] \| Equity securities [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	329.4	193.6	296.4	199.7
Acquisition		45.2		45.2
Realized and unrealized gains (losses) included in investment and other income (losses), net	(11.0)	(4.5)	(10.0)	6.9
Purchases	36.4	25.7	68.4	35.4
Sales and settlements	(2.1)		(3.3)	(1.4)
Sales		(0.3)
Deconsolidation			0.0
Transfers into Level 3	0.0		0.0	0.1
Transfers out of Level 3	0.0		0.0	(25.4)
Foreign exchange revaluation	(1.1)	(2.1)	0.1	(2.9)
Balance at End of Period - assets	351.6	257.6	351.6	257.6
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2020	(11.0)	(4.8)	(10.0)	7.0
CIPs [Member] \| Level 3 [Member] \| Debt Securities [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	83.4	100.9	131.4	118.0
Acquisition		39.7		39.7
Realized and unrealized gains (losses) included in investment and other income (losses), net	(6.3)	(3.8)	(14.3)	(11.0)
Purchases	3.6	9.4	4.3	9.4
Sales and settlements	(0.7)		(1.1)	(20.2)
Sales		(14.2)
Deconsolidation			(40.7)
Transfers into Level 3	2.2		2.2	0.0
Transfers out of Level 3	(1.0)		(1.0)	(3.6)
Foreign exchange revaluation	(0.3)	(0.3)	0.1	(0.6)
Balance at End of Period - assets	80.9	131.7	80.9	131.7
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2020	(6.3)	(3.2)	(14.3)	(3.9)
CIPs [Member] \| Level 3 [Member] \| Real Estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	219.9	0.0	152.7	0.0
Acquisition		0.0		0.0
Realized and unrealized gains (losses) included in investment and other income (losses), net	1.8	3.4	(0.9)	3.4
Purchases	77.3	25.6	141.6	25.6
Sales and settlements	0.0		0.0	0.0
Sales		0.0
Deconsolidation			0.0
Transfers into Level 3	0.0		0.0	0.0
Transfers out of Level 3	0.0		0.0	0.0
Foreign exchange revaluation	(11.0)	0.1	(5.4)	0.1
Balance at End of Period - assets	288.0	29.1	288.0	29.1
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2020	1.8	3.4	(0.9)	3.4
CIPs [Member] \| Level 3 [Member] \| Loans [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	14.0	39.2	16.6	32.3
Acquisition		0.0		0.0
Realized and unrealized gains (losses) included in investment and other income (losses), net	(1.3)	(0.5)	(1.4)	(1.5)
Purchases	0.0	0.0	0.0	8.2
Sales and settlements	(1.7)		(4.2)	(0.6)
Sales		(0.3)
Deconsolidation			0.0
Transfers into Level 3	0.0		0.0	0.0
Transfers out of Level 3	0.0		0.0	0.0
Foreign exchange revaluation	0.0	0.0	0.0	0.0
Balance at End of Period - assets	11.0	38.4	11.0	38.4
Change in unrealized gains (losses) included in net income relating to assets held at March 31, 2020	$ (1.7)	$ (0.2)	$ (1.4)	$ (0.9)

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

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