Related Party Transaction (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Cash and cash equivalents	$ 835.5	$ 727.4	$ 740.5	$ 762.0
Investment Advisory Fees	3,236.9	3,138.5	2,720.9
Distribution and Servicing Fees	771.6	780.3	645.5
Performance Fees	58.8	37.9	26.1
Revenue, Other Financial Services	109.7	135.5	95.2
Revenues	4,177.0	4,092.2	3,487.7
Unsettled fund receivables	550.1	444.4
Accounts Receivable, Net, Current	449.4	424.4
Investments	363.9	283.7
Separate Account Assets	1,153.6	1,243.5
Other Assets, Current	94.5	150.0
Investments	246.8	200.8
Other Assets, Noncurrent	26.8	17.9
Assets	17,492.4	19,347.0
Unsettled fund payables	552.5	439.6
Other Liabilities, Current	824.7	841.5
Other Liabilities, Noncurrent	306.2	297.3
Liabilities	8,443.4	10,209.4
Affiliated Entity [Member]
Cash and cash equivalents	223.2	257.7
Investment Advisory Fees	2,767.5	2,694.3	2,349.0
Distribution and Servicing Fees	752.1	779.7	645.0
Performance Fees	50.0	33.0	22.1
Revenue, Other Financial Services	101.6	128.9	93.3
Revenues	3,671.2	3,635.9	3,109.4
Unsettled fund receivables	131.5	178.8
Accounts Receivable, Net, Current	258.3	247.8
Investments	328.2	248.9
Separate Account Assets	1,153.2	1,243.1
Other Assets, Current	30.5	22.1
Investments	234.6	184.5
Other Assets, Noncurrent	2.2	1.9
Assets	2,361.7	2,384.8
Unsettled fund payables	266.0	205.0
Other Liabilities, Current	66.1	63.5
Other Liabilities, Noncurrent	189.7	191.6
Liabilities	$ 521.8	$ 460.1