Investments Offsetting for Derivatives and Securities Lending Balances (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Securities Lending [Abstract]
Securities Loaned	$ 236.5	$ 452.8
Securities Lending Agreements Offset in Balance Sheet	0	0
Investment Securities Invested from Securities Lending Transactions	(153.6)	(452.8)
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	153.6	455.8
Securities Lending Asset Offsets [Abstract]
Securities Received as Collateral	(82.9)	0
Cash collateral offset against securities purchased	153.6	452.8
Net Exposure on Securities Purchased	0	0
Securities Lending Liability Offsets [Abstract]
Cash Collateral Pledged on Securities Lending Transactions	0	0
Securities Lending Agreements, Net	0	3.0
Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	36.4	81.6
Derivative Asset, Fair Value, Gross Liability	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	36.4	81.6
Financial Instruments Pledged as Collateral from Counterparty	(35.8)	(72.9)
Derivative, Collateral, Obligation to Return Cash	(0.3)	0
Derivative Asset, Fair Value, Net	0.3	8.7
Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	137.6	170.5
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	137.6	170.5
Financial Instruments Pledged as Collateral to Counterparty	(104.6)	(129.8)
Derivative, Collateral, Right to Reclaim Cash	0	1.8
Derivative Liability, Fair Value, Net	33.0	38.9
Financial Assets [Abstract]
Financial Assets Subject to Master Netting Arrangemetns Offset in the Balance Sheet	0	0
Total Cash Received as Collateral from Contracts with Master Netting Arrangements	(153.9)	(452.8)
Financial Assets Subject to Master Netting Arrangements Presented in Balance Sheet	272.9	534.4
Financial Assets Subject to Master Netting Arrangements, Gross	272.9	534.4
Financial Assets Subject to Master Netting Arrangements, Net	0.3	8.7
Total Financial Instruments Received as Collateral	(118.7)	(72.9)
Financial Liabilities [Abstract]
Financial Liabilities Subject to Master Netting Arrangements, Gross	291.2	626.3
Financial Liabilities Subject to Master Netting Arrangements Offset in the Balance Sheet	0	0
Financial Liabilities Subject to Master Netting Arrangements Presented in Balance Sheet	291.2	626.3
Total Financial Instruments Pledged as Collateral to Counterparty	(258.2)	(582.6)
Total Cash Pledged as Collateral on Contracts with Master Netting Arrangements	0	(1.8)
Financial Liabilities Subject to Master Netting Arrangements, Net	$ 33.0	$ 41.9