Consolidated Investment Products - Schedule of Changes in Level 3 Assets of CIPs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule Of Consolidated Investment Products [Line Items]
Transfers into Level 3			$ 0.0	$ 0.0
Consolidated Investment Products [Member]
Schedule Of Consolidated Investment Products [Line Items]
Transfers into Level 3	$ 0.0	$ 0.0	0.1	0.0
Transfers out of Level 3	0.0	0.0	(29.0)	0.0
Consolidated Investment Products [Member] \| Level 3 [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	456.8	300.6	350.0	296.1
Acquisition			84.9
Realized and unrealized gains (losses) included in investment and other income (losses), net	8.2	5.7	6.0	25.3
Purchases	39.8	5.1	118.4	38.2
Sales	(29.4)	(2.7)	(50.5)	(55.3)
Settlements	(13.8)	(0.5)	(14.9)	(0.5)
Consolidation		7.0		7.0
Transfers into Level 3			0.1
Transfers out of Level 3			(29.0)
Foreign exchange revaluation	0.6	(4.8)	(2.8)	(0.4)
Balance at End of Period - assets	462.2	310.4	462.2	310.4
Change in unrealized gains (losses) included in net income relating to assets held at June 30, 2019	(4.7)	1.1	1.0	15.4
Consolidated Investment Products [Member] \| Level 3 [Member] \| Equity securities [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	257.6	188.4	199.7	160.7
Acquisition			45.2
Realized and unrealized gains (losses) included in investment and other income (losses), net	0.5	5.4	7.4	22.9
Purchases	38.8	0.1	74.2	22.7
Sales	(29.4)	(2.6)	(29.8)	(17.5)
Settlements	0.0	0.0	(1.0)	0.0
Consolidation		0.0		0.0
Transfers into Level 3			0.1
Transfers out of Level 3			(25.4)
Foreign exchange revaluation	0.1	(3.8)	(2.8)	(1.3)
Balance at End of Period - assets	267.6	187.5	267.6	187.5
Change in unrealized gains (losses) included in net income relating to assets held at June 30, 2019	(13.3)	0.9	(6.3)	17.5
Consolidated Investment Products [Member] \| Level 3 [Member] \| Debt Securities [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	131.7	112.2	118.0	135.4
Acquisition			39.7
Realized and unrealized gains (losses) included in investment and other income (losses), net	8.6	0.3	(2.4)	2.4
Purchases	0.0	0.0	9.4	10.5
Sales	0.0	(0.1)	(20.1)	(37.8)
Settlements	0.0	(0.5)	(0.1)	(0.5)
Consolidation		0.0		0.0
Transfers into Level 3			0.0
Transfers out of Level 3			(3.6)
Foreign exchange revaluation	0.3	(1.0)	(0.3)	0.9
Balance at End of Period - assets	140.6	110.9	140.6	110.9
Change in unrealized gains (losses) included in net income relating to assets held at June 30, 2019	8.8	0.2	4.9	(2.1)
Consolidated Investment Products [Member] \| Level 3 [Member] \| Loans [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	38.4	0.0	32.3	0.0
Acquisition			0.0
Realized and unrealized gains (losses) included in investment and other income (losses), net	(0.7)	0.0	(2.2)	0.0
Purchases	1.0	5.0	9.2	5.0
Sales	0.0	0.0	(0.6)	0.0
Settlements	(13.8)	0.0	(13.8)	0.0
Consolidation		7.0		7.0
Transfers into Level 3			0.0
Transfers out of Level 3			0.0
Foreign exchange revaluation	0.0	0.0	0.0	0.0
Balance at End of Period - assets	24.9	12.0	24.9	12.0
Change in unrealized gains (losses) included in net income relating to assets held at June 30, 2019	0.0	$ 0.0	(0.8)	$ 0.0
Consolidated Investment Products [Member] \| Level 3 [Member] \| Real Estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of period - assets	29.1		0.0
Acquisition			0.0
Realized and unrealized gains (losses) included in investment and other income (losses), net	(0.2)		3.2
Purchases	0.0		25.6
Sales	0.0		0.0
Settlements	0.0		0.0
Transfers into Level 3			0.0
Transfers out of Level 3			0.0
Foreign exchange revaluation	0.2		0.3
Balance at End of Period - assets	29.1		29.1
Change in unrealized gains (losses) included in net income relating to assets held at June 30, 2019	$ (0.2)		$ 3.2

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Amount of increase (decrease) of assets measured at fair value on a recurring basis using unobservable inputs acquired during the period.

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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 (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (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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