Noncontrolling Interests (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012 Item	Sep. 30, 2013 Seeded investment products	Sep. 30, 2012 Seeded investment products	Sep. 30, 2013 Seeded investment products	Sep. 30, 2012 Seeded investment products	Sep. 30, 2013 Subsidiaries	Sep. 30, 2012 Subsidiaries	Sep. 30, 2013 Subsidiaries	Sep. 30, 2012 Subsidiaries	Sep. 30, 2013 INTECH and Perkins employees	Dec. 31, 2012 INTECH and Perkins employees
Noncontrolling interests in net income consist
Nonredeemable noncontrolling interests					$ 2.0	$ 0.6	$ 2.1	$ 1.7	$ 0.3	$ 0.5	$ 0.8	$ 1.4
Redeemable noncontrolling interests									0.7	1.5	2.8	4.9
Total noncontrolling interests in net income	3.0	2.6	5.7	8.0
Number of subsidiaries having employee ownership interest that are not subject to redemption rights				2
Total nonredeemable noncontrolling interests	38.1		38.1		33.9	12.6	33.9	12.6					4.2	4.8
Rollforward of noncontrolling interests in consolidated seed investment products
Balance at beginning of period			17.2		26.1	35.6	12.4	29.2					4.2	4.8
Changes in market value					2.0	0.6	2.1	1.7
Changes in ownership					5.8	(23.6)	19.4	(18.3)
Balance at end of period	$ 38.1		$ 38.1		$ 33.9	$ 12.6	$ 33.9	$ 12.6					$ 4.2	$ 4.8