Commitments and Contingent Liabilities (Commitments and Guarantees) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013 credit_facilities	Dec. 31, 2012
Commitments And Contingent Liabilities [Line Items]
Undrawn outstanding letters of credit in favor of third parties	$ 223.7	$ 45.4
Undrawn outstanding letters of credit between subsidiaries	916.5	763.5
Number of credit facilities	7
Syndicated revolving credit facility capacity	850.0
Insurance Treaty Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	860.6	686.0
Insurance Treaty Guarantee Net Of Assets Held In Trust [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	669.9	463.5
Guarantee Of Borrowed Securities [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	87.5
Guarantee Of Subsidiary Collateral Financing [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	300.0
Letter of Credit [Member]
Commitments And Contingent Liabilities [Line Items]
Number of credit facilities	6
Facility Capacity	894.3
Commitment Period 2033 [Member] | Statutory Reserve Support Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Maximum potential obligation, Undiscounted	1,350.0
Commitment Period 2035 [Member] | Statutory Reserve Support Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Maximum potential obligation, Undiscounted	560.0
Limited Partnerships [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	241.9	176.7
Commercial Mortgage Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	23.1	22.2
Bank Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	$ 58.2	$ 68.5