Investments Offsetting for Derivatives and Securities Lending Balances (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Securities Lending [Abstract]
Securities Lending Agreements Offset in Balance Sheet	$ 0	$ 0
Securities Lending Asset Offsets [Abstract]
Securities Loaned	254.4	452.8
Cash collateral offset against securities purchased	(140.5)	(452.8)
Net Exposure on Securities Purchased	0	0
Securities Lending Liability Offsets [Abstract]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	140.5	455.8
Investment Securities Invested from Securities Lending Transactions	(140.5)	(452.8)
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	24.5	81.6
Derivative Asset, Fair Value, Gross Liability	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	24.5	81.6
Financial Instruments Pledged as Collateral from Counterparty	(22.8)	(72.9)
Cash collateral from counterparties	(1.3)	0
Derivative Asset, Fair Value, Net	0.4	8.7
Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	142.9	170.5
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	142.9	170.5
Financial Instruments Pledged as Collateral to Counterparty	(112.3)	(129.8)
Derivative, Collateral, Right to Reclaim Cash	0	(1.8)
Derivative Liability, Fair Value, Net	30.6	38.9
Financial Assets [Abstract]
Financial Assets Subject to Master Netting Arrangements, Gross	278.9	534.4
Financial Assets Subject to Master Netting Arrangemetns Offset in the Balance Sheet	0	0
Financial Assets Subject to Master Netting Arrangements Presented in Balance Sheet	278.9	534.4
Total Financial Instruments Received as Collateral	(136.7)	(72.9)
Total Cash Received as Collateral from Contracts with Master Netting Arrangements	(141.8)	(452.8)
Financial Assets Subject to Master Netting Arrangements, Net	0.4	8.7
Financial Liabilities [Abstract]
Financial Liabilities Subject to Master Netting Arrangements, Gross	283.4	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	283.4	626.3
Total Financial Instruments Pledged as Collateral to Counterparty	(252.8)	(582.6)
Total Cash Pledged as Collateral on Contracts with Master Netting Arrangements	0	(1.8)
Financial Liabilities Subject to Master Netting Arrangements, Net	$ 30.6	$ 41.9