Investments Repurchase Agreement, Securities Lending, VIEs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Payables under securities loan agreement, including collateral held	$ 483.9	$ 483.9	$ 634.8
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Gain (loss) on sale of VIE	38.7
Number of tranches for VIE sales transaction		2
Proceeds from sale of VIE		190.0
Collateralized loan obligations | Not a primary beneficiary of the VIE
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Carrying value of VIE	1.1	1.1	0.9
Securities pledged as collateral
Available-for-sale Securities Including Securities Pledged [Line Items]
Fair value of loaned securities	306.4	306.4	515.8
Cash collateral, included in Payables
Available-for-sale Securities Including Securities Pledged [Line Items]
Payables under securities loan agreement, including collateral held	315.3	315.3	524.8
Sold variable interest entity | Not a primary beneficiary of the VIE
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Carrying value of VIE	$ 331.9	$ 331.9