Investments - Repurchase Agreement, Securities Lending, VIEs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	5 Months Ended
	Jun. 30, 2012	Oct. 29, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Collateralized loan obligations Not a primary beneficiary of the VIE	Dec. 31, 2011 Collateralized loan obligations Not a primary beneficiary of the VIE	Dec. 31, 2012 Securities pledged as collateral	Dec. 31, 2011 Securities pledged as collateral	Dec. 31, 2012 Cash collateral, included in Payables	Dec. 31, 2011 Cash collateral, included in Payables	Mar. 31, 2012 Sold variable interest entity Not a primary beneficiary of the VIE	Dec. 31, 2012 Notes Receivable
Available-for-sale Securities Including Securities Pledged [Line Items]
Securities Loaned, Fair Value Disclosure			$ 180.2	$ 515.8			$ 180.2	$ 515.8
Securities Received as Collateral			186.1	524.8
Payables under securities loan agreement, including collateral held			353.2	634.8					186.1	524.8
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Carrying value of VIE					1.3	0.9					331.9
Gain (loss) on sale of VIE	(38.7)
Number of tranches for VIE sales transaction		2
Proceeds from sale of VIE												$ 23.0