Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Capital in Excess of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Stockholders' Equity [Member]	Nonredeemable Noncontrolling Interests [Member]
Stockholders' Equity [Roll Forward]
Adoption of new accounting guidance | ASU 2014-09 and 2016-01 [Member]	$ 14.9			$ 22.9		$ 14.9
Adoption of new accounting guidance | ASU 2016-01 [Member]					$ (8.0)
Beginning balance - Shares at Sep. 30, 2018		519.1
Beginning balance at Sep. 30, 2018		$ 51.9	$ 0.0	10,217.9	(370.6)	9,899.2
Beginning balance at Sep. 30, 2018							$ 308.7
Beginning balance at Sep. 30, 2018	10,207.9
Stockholders' Equity [Roll Forward]
Net income (loss)	275.9			275.9		275.9
Nonredeemable noncontrolling interests	(0.5)						(0.5)
Net income (loss)	275.4
Other comprehensive income (loss)	(15.0)				(15.0)	(15.0)
Dividends declared on common stock	(133.8)			(133.8)		(133.8)
Repurchase of common stock - Shares		(10.7)
Repurchase of common stock - Amount	(326.9)	$ (1.1)	(30.8)	(295.0)		(326.9)
Issuance of common stock - Shares		3.1
Issuance of common stock - Amount	33.9	$ 0.3	33.6			33.9
Stock-based compensation	(2.8)		(2.8)			(2.8)
Net subscriptions and other	23.1						23.1
Ending balance - Shares at Dec. 31, 2018		511.5
Ending balance at Dec. 31, 2018		$ 51.1	0.0	10,087.9	(393.6)	9,745.4
Ending balance at Dec. 31, 2018							331.3
Ending balance at Dec. 31, 2018	10,076.7
Beginning balance - Shares at Sep. 30, 2019		499.3
Beginning balance at Sep. 30, 2019	9,906.5	$ 49.9	0.0	10,288.2	(431.6)	9,906.5
Beginning balance at Sep. 30, 2019	717.7						717.7
Beginning balance at Sep. 30, 2019	10,624.2
Stockholders' Equity [Roll Forward]
Net income (loss)	350.5			350.5		350.5
Nonredeemable noncontrolling interests	(11.4)						(11.4)
Net income (loss)	339.1
Other comprehensive income (loss)	55.4				55.4	55.4
Dividends declared on common stock	(135.3)			(135.3)		(135.3)
Repurchase of common stock - Shares		(4.6)
Repurchase of common stock - Amount	(123.6)	$ (0.4)	(27.8)	(95.4)		(123.6)
Issuance of common stock - Shares		2.9
Issuance of common stock - Amount	29.0	$ 0.3	28.7			29.0
Stock-based compensation	(0.9)		(0.9)			(0.9)
Net subscriptions and other	28.2						28.2
Deconsolidation of investment products	(0.7)						(0.7)
Acquisition	19.0						19.0
Ending balance - Shares at Dec. 31, 2019		497.6
Ending balance at Dec. 31, 2019	10,081.6	$ 49.8	$ 0.0	$ 10,408.0	$ (376.2)	$ 10,081.6
Ending balance at Dec. 31, 2019	752.8						$ 752.8
Ending balance at Dec. 31, 2019	$ 10,834.4