Related Party Transaction (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Related Party Transaction [Line Items]
Investment management fees	$ 914.4	$ 790.6	$ 2,644.5	$ 2,309.6
Performance fees	5.1	3.0	47.2	39.0
Other	31.6	24.3	85.1	83.0
Total operating revenues	1,171.8	1,013.9	3,419.5	3,003.7
Affiliated asset balances:
Cash equivalents	1,174.5	880.1	1,174.5	880.1	835.5	835.5	727.4
Unsettled fund receivables	1,003.1		1,003.1		550.1
Accounts receivable	475.6		475.6		449.4
Assets held for policyholders	1,449.0		1,449.0		1,153.6
Assets held for sale	106.7		106.7		0
Other assets	107.6		107.6		146.8
Total assets	18,763.5		18,763.5		17,492.4
Affiliated liability balances:
Accrued compensation and benefits	565.5		565.5		609.8
Accounts payable and accrued expenses	660.3		660.3		626.4
Unsettled fund payables	993.8		993.8		552.5
Total liabilities	9,644.1		9,644.1		8,443.4
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Investment management fees	811.3	697.1	2,343.1	2,030.4
Service and distribution fees	216.7	181.6	630.0	557.4
Performance fees	3.6	2.0	37.6	33.9
Other	26.1	23.6	78.9	76.7
Total operating revenues	1,057.7	904.3	3,089.6	2,698.4
Affiliated asset balances:
Cash equivalents	319.6		319.6		223.2
Unsettled fund receivables	377.5		377.5		131.5
Accounts receivable	300.1		300.1		258.3
Investments	648.4		648.4		562.8
Assets held for policyholders	1,448.7		1,448.7		1,153.2
Assets held for sale	4.5		4.5		0
Other assets	9.2		9.2		32.7
Total assets	3,108.0		3,108.0		2,361.7
Affiliated liability balances:
Accrued compensation and benefits	149.5		149.5		234.3
Accounts payable and accrued expenses	19.0		19.0		21.5
Unsettled fund payables	428.6		428.6		266.0
Total liabilities	$ 597.1		$ 597.1		$ 521.8