Commitments and Contingent Liabilities (Details 1) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013 credit_facilities	Dec. 31, 2012
Commitments And Contingent Liabilities [Line Items]
Undrawn outstanding letters of credit in favor of third parties	$ 76.2	$ 45.4
Undrawn outstanding letters of credit between subsidiaries	861.3	763.5
Number of credit facilities	5
Syndicated revolving credit facility capacity	850.0
Insurance Treaty Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	815.2	686.0
Insurance Treaty Guarantee Net Of Assets Held In Trust [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	628.1	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	4
Facility Capacity	690.0
Commitment Period 2033 [Member] | Statutory Reserve Support Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Maximum potential obligation, Undiscounted	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	241.8	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	42.5	22.2
Bank Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	$ 74.5	$ 68.5