Investments Offsetting for Derivatives and Securities Lending Balances (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Securities Lending [Abstract]
Securities Loaned	$ 234.0	$ 452.8
Securities Lending Agreements Offset in Balance Sheet	0	0
Securities Lending Asset Offsets [Abstract]
Securities Received as Collateral	0	0
Cash collateral offset against securities purchased	234.0	452.8
Net Exposure on Securities Purchased	0	0
Securities Lending Liability Offsets [Abstract]
Cash Collateral Pledged on Securities Lending Transactions	0	0
Cash Collateral for Borrowed Securities	236.3	455.8
Securities Lending Agreements, Net	2.3	3.0
Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	62.1	81.6
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	62.1	81.6
Financial Instruments Pledged as Collateral from Counterparty	(53.6)	(72.9)
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset, Fair Value, Net	8.5	8.7
Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	157.4	170.5
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	157.4	170.5
Financial Instruments Pledged as Collateral to Counterparty	(117.7)	(129.8)
Derivative, Collateral, Right to Reclaim Cash	1.8	1.8
Derivative Liability, Fair Value, Net	37.9	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	(234.0)	(452.8)
Financial Assets Subject to Master Netting Arrangements Presented in Balance Sheet	296.1	534.4
Financial Assets Subject to Master Netting Arrangements, Gross	296.1	534.4
Financial Assets Subject to Master Netting Arrangements, Net	8.5	8.7
Total Financial Instruments Received as Collateral	(53.6)	(72.9)
Financial Liabilities [Abstract]
Financial Liabilities Subject to Master Netting Arrangements, Gross	393.7	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	393.7	626.3
Total Financial Instruments Pledged as Collateral to Counterparty	(351.7)	(582.6)
Total Cash Pledged as Collateral on Contracts with Master Netting Arrangements	(1.8)	(1.8)
Financial Liabilities Subject to Master Netting Arrangements, Net	$ 40.2	$ 41.9