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				275.9		275.9
Nonredeemable noncontrolling interests							(0.5)
Net income	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, 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	643.4
Nonredeemable noncontrolling interests	(7.9)
Other comprehensive income (loss)	(7.9)
Ending balance - Shares at Mar. 31, 2019		507.7
Ending balance at Mar. 31, 2019		$ 50.8	0.0	10,219.5	(386.5)	9,883.8
Ending balance at Mar. 31, 2019							615.3
Ending balance at Mar. 31, 2019	10,499.1
Beginning balance - Shares at Dec. 31, 2018		511.5
Beginning balance at Dec. 31, 2018		$ 51.1	0.0	10,087.9	(393.6)	9,745.4
Beginning balance at Dec. 31, 2018							331.3
Beginning balance at Dec. 31, 2018	10,076.7
Stockholders' Equity [Roll Forward]
Net income	367.5			367.5		367.5
Nonredeemable noncontrolling interests	(7.4)						(7.4)
Net income	360.1
Other comprehensive income (loss)	7.1				7.1	7.1
Dividends declared on common stock	(132.6)			(132.6)		(132.6)
Repurchase of common stock - Shares		(4.6)
Repurchase of common stock - Amount	(144.7)	$ (0.4)	(41.0)	(103.3)		(144.7)
Issuance of common stock - Shares		0.8
Issuance of common stock - Amount	13.4	$ 0.1	13.3			13.4
Stock-based compensation	27.7		27.7			27.7
Net subscriptions and other	51.0						51.0
Deconsolidation of investment products	24.3						24.3
Acquisition	216.1						216.1
Ending balance - Shares at Mar. 31, 2019		507.7
Ending balance at Mar. 31, 2019		$ 50.8	0.0	10,219.5	(386.5)	9,883.8
Ending balance at Mar. 31, 2019							615.3
Ending balance at Mar. 31, 2019	10,499.1
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				350.5		350.5
Nonredeemable noncontrolling interests							(11.4)
Net income	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		$ 49.8	0.0	10,408.0	(376.2)	10,081.6
Ending balance at Dec. 31, 2019							752.8
Ending balance at Dec. 31, 2019	10,834.4
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	429.6
Nonredeemable noncontrolling interests	(3.2)
Other comprehensive income (loss)	(44.5)
Ending balance - Shares at Mar. 31, 2020		495.3
Ending balance at Mar. 31, 2020	9,903.4	$ 49.5	0.0	10,330.0	(476.1)	9,903.4
Ending balance at Mar. 31, 2020	850.1						850.1
Ending balance at Mar. 31, 2020	10,753.5
Beginning balance - Shares at Dec. 31, 2019		497.6
Beginning balance at Dec. 31, 2019		$ 49.8	0.0	10,408.0	(376.2)	10,081.6
Beginning balance at Dec. 31, 2019							752.8
Beginning balance at Dec. 31, 2019	10,834.4
Stockholders' Equity [Roll Forward]
Net income	79.1			79.1		79.1
Nonredeemable noncontrolling interests	8.2						8.2
Net income	87.3
Other comprehensive income (loss)	(99.9)				(99.9)	(99.9)
Dividends declared on common stock	(134.6)			(134.6)		(134.6)
Repurchase of common stock - Shares		(2.9)
Repurchase of common stock - Amount	(64.6)	$ (0.4)	(41.7)	(22.5)		(64.6)
Issuance of common stock - Shares		0.6
Issuance of common stock - Amount	16.0	$ 0.1	15.9			16.0
Stock-based compensation	25.8		25.8			25.8
Net subscriptions and other	89.7						89.7
Deconsolidation of investment products	(0.6)						(0.6)
Ending balance - Shares at Mar. 31, 2020		495.3
Ending balance at Mar. 31, 2020	9,903.4	$ 49.5	$ 0.0	$ 10,330.0	$ (476.1)	$ 9,903.4
Ending balance at Mar. 31, 2020	850.1						$ 850.1
Ending balance at Mar. 31, 2020	$ 10,753.5