Taxation (Income Before Taxes) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	$ 908.1	$ 833.8	$ 357.5
Consolidated Investment Products [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	(87.4)	180.7	(113.9)
US [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	579.4	271.0	(3.2)
US [Member] | Separate Institutional Accounts [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	486.4	173.8	102.3
US [Member] | Consolidated Investment Products [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	93.0	97.2	(105.5)
Foreign [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	328.7	562.8	360.7
Foreign [Member] | Separate Institutional Accounts [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	509.1	479.3	369.1
Foreign [Member] | Consolidated Investment Products [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	$ (180.4)	$ 83.5	$ (8.4)