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]
Beginning balance - Shares at Sep. 30, 2019		499.3
Beginning balance at Sep. 30, 2019		$ 49.9	$ 0.0	$ 10,288.2	$ (431.6)	$ 9,906.5
Beginning balance at Sep. 30, 2019							$ 717.7
Beginning balance at Sep. 30, 2019	$ 10,624.2
Stockholders' Equity [Roll Forward]
Net income	350.5			350.5		350.5
Nonredeemable noncontrolling interests	(11.4)						(11.4)
Net income	339.1
Other comprehensive income	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
Stockholders' Equity [Roll Forward]
Adoption of new accounting guidance | ASU 2016-13	(3.3)			(3.3)		(3.3)
Beginning balance - Shares at Sep. 30, 2020		495.1
Beginning balance at Sep. 30, 2020	10,114.5	$ 49.5	0.0	10,472.6	(407.6)	10,114.5
Beginning balance at Sep. 30, 2020	859.3						859.3
Beginning balance at Sep. 30, 2020	10,973.8
Stockholders' Equity [Roll Forward]
Net income	345.3			345.3		345.3
Nonredeemable noncontrolling interests	30.8						30.8
Net income	376.1
Other comprehensive income	92.9				92.9	92.9
Dividends declared on common stock	(144.0)			(144.0)		(144.0)
Repurchase of common stock - Shares		(2.1)
Repurchase of common stock - Amount	(45.6)	$ (0.2)	(39.9)	(5.5)		(45.6)
Issuance of common stock - Shares		12.5
Issuance of common stock - Amount	33.7	$ 1.2	32.5			33.7
Stock-based compensation	7.4		7.4			7.4
Net subscriptions and other	129.6						129.6
Ending balance - Shares at Dec. 31, 2020		505.5
Ending balance at Dec. 31, 2020	10,400.9	$ 50.5	$ 0.0	$ 10,665.1	$ (314.7)	$ 10,400.9
Ending balance at Dec. 31, 2020	1,019.7						$ 1,019.7
Ending balance at Dec. 31, 2020	$ 11,420.6