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 2016-09 [Member]	$ 0.4		$ 2.1	$ (1.7)		$ 0.4
Beginning balance - Shares at Sep. 30, 2017		554.9
Beginning balance at Sep. 30, 2017		$ 55.5	0.0	12,849.3	$ (284.8)	12,620.0
Beginning balance at Sep. 30, 2017							$ 315.8
Beginning balance at Sep. 30, 2017	12,935.8
Stockholders' Equity [Roll Forward]
Net income (loss)				(583.3)		(583.3)
Nonredeemable noncontrolling interests							(0.1)
Net income (loss)	(583.4)
Other comprehensive income (loss)	18.2				18.2	18.2
Dividends declared on common stock	(127.4)			(127.4)		(127.4)
Repurchase of common stock - Shares		(4.6)
Repurchase of common stock - Amount	(200.0)	$ (0.5)	(32.1)	(167.4)		(200.0)
Issuance of common stock - Shares		2.1
Issuance of common stock - Amount	27.8	$ 0.2	27.6			27.8
Stock-based compensation	2.4		2.4			2.4
Net subscriptions (distributions) and other	7.0						7.0
Consolidation (deconsolidation) of investment product	(0.3)						(0.3)
Ending balance - Shares at Dec. 31, 2017		552.4
Ending balance at Dec. 31, 2017		$ 55.2	0.0	11,969.5	(266.6)	11,758.1
Ending balance at Dec. 31, 2017							322.4
Ending balance at Dec. 31, 2017	12,080.5
Beginning balance - Shares at Sep. 30, 2017		554.9
Beginning balance at Sep. 30, 2017		$ 55.5	0.0	12,849.3	(284.8)	12,620.0
Beginning balance at Sep. 30, 2017							315.8
Beginning balance at Sep. 30, 2017	12,935.8
Stockholders' Equity [Roll Forward]
Net income (loss)	261.9
Nonredeemable noncontrolling interests	22.8
Other comprehensive income (loss)	(58.0)
Ending balance - Shares at Jun. 30, 2018		529.1
Ending balance at Jun. 30, 2018		$ 52.9	0.0	10,153.5	(342.9)	9,863.5
Ending balance at Jun. 30, 2018							326.9
Ending balance at Jun. 30, 2018	10,190.4
Stockholders' Equity [Roll Forward]
Adoption of new accounting guidance | ASU 2018-02 [Member]	0.0			0.1	(0.1)	0.0
Beginning balance - Shares at Dec. 31, 2017		552.4
Beginning balance at Dec. 31, 2017		$ 55.2	0.0	11,969.5	(266.6)	11,758.1
Beginning balance at Dec. 31, 2017							322.4
Beginning balance at Dec. 31, 2017	12,080.5
Stockholders' Equity [Roll Forward]
Net income (loss)				443.2		443.2
Nonredeemable noncontrolling interests							24.5
Net income (loss)	467.7
Other comprehensive income (loss)	7.0				7.0	7.0
Dividends declared on common stock	(1,762.0)			(1,762.0)		(1,762.0)
Repurchase of common stock - Shares		(11.1)
Repurchase of common stock - Amount	(432.6)	$ (1.1)	(44.6)	(386.9)		(432.6)
Issuance of common stock - Shares		0.4
Issuance of common stock - Amount	13.8	$ 0.1	13.7			13.8
Stock-based compensation	30.9		30.9			30.9
Net subscriptions (distributions) and other	(1.3)						(1.3)
Consolidation (deconsolidation) of investment product	(1.8)						(1.8)
Ending balance - Shares at Mar. 31, 2018		541.7
Ending balance at Mar. 31, 2018		$ 54.2	0.0	10,263.9	(259.7)	10,058.4
Ending balance at Mar. 31, 2018							343.8
Ending balance at Mar. 31, 2018	10,402.2
Stockholders' Equity [Roll Forward]
Net income (loss)	402.0			402.0		402.0
Nonredeemable noncontrolling interests	(1.6)						(1.6)
Net income (loss)	400.4
Other comprehensive income (loss)	(83.2)				(83.2)	(83.2)
Dividends declared on common stock	(122.3)			(122.3)		(122.3)
Repurchase of common stock - Shares		(13.4)
Repurchase of common stock - Amount	(446.2)	$ (1.4)	(54.7)	(390.1)		(446.2)
Issuance of common stock - Amount	0.8		0.8			0.8
Stock-based compensation	27.0		27.0			27.0
Net subscriptions (distributions) and other	(19.8)						(19.8)
Consolidation (deconsolidation) of investment product	4.5						4.5
Acquisition - Shares		0.8
Acquisition	27.0	$ 0.1	26.9			27.0
Ending balance - Shares at Jun. 30, 2018		529.1
Ending balance at Jun. 30, 2018		$ 52.9	0.0	10,153.5	(342.9)	9,863.5
Ending balance at Jun. 30, 2018							326.9
Ending balance at Jun. 30, 2018	10,190.4
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	9,899.2	$ 51.9	0.0	10,217.9	(370.6)	9,899.2
Beginning balance at Sep. 30, 2018	308.7						308.7
Beginning balance at Sep. 30, 2018	10,207.9
Stockholders' Equity [Roll Forward]
Net income (loss)				275.9		275.9
Nonredeemable noncontrolling interests							(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 (distributions) 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	9,899.2	$ 51.9	0.0	10,217.9	(370.6)	9,899.2
Beginning balance at Sep. 30, 2018	308.7						308.7
Beginning balance at Sep. 30, 2018	10,207.9
Stockholders' Equity [Roll Forward]
Net income (loss)	889.3
Nonredeemable noncontrolling interests	0.7
Other comprehensive income (loss)	(2.3)
Ending balance - Shares at Jun. 30, 2019		504.1
Ending balance at Jun. 30, 2019	9,911.0	$ 50.4	0.0	10,241.5	(380.9)	9,911.0
Ending balance at Jun. 30, 2019	654.8						654.8
Ending balance at Jun. 30, 2019	10,565.8
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 (loss)				367.5		367.5
Nonredeemable noncontrolling interests							(7.4)
Net income (loss)	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 (distributions) and other	51.0						51.0
Consolidation (deconsolidation) of investment product	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
Stockholders' Equity [Roll Forward]
Net income (loss)	245.9			245.9		245.9
Nonredeemable noncontrolling interests	8.6						8.6
Net income (loss)	254.5
Other comprehensive income (loss)	5.6				5.6	5.6
Dividends declared on common stock	(131.7)			(131.7)		(131.7)
Repurchase of common stock - Shares		(3.6)
Repurchase of common stock - Amount	(121.3)	$ (0.4)	(28.7)	(92.2)		(121.3)
Issuance of common stock - Shares		0.0
Issuance of common stock - Amount	0.6	$ 0.0	0.6			0.6
Stock-based compensation	28.1		28.1			28.1
Net subscriptions (distributions) and other	30.9						30.9
Ending balance - Shares at Jun. 30, 2019		504.1
Ending balance at Jun. 30, 2019	9,911.0	$ 50.4	$ 0.0	$ 10,241.5	$ (380.9)	$ 9,911.0
Ending balance at Jun. 30, 2019	654.8						$ 654.8
Ending balance at Jun. 30, 2019	$ 10,565.8