Commitments and Contingent Liabilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014 credit_facilities	Dec. 31, 2013
Commitments And Contingent Liabilities [Line Items]
Undrawn outstanding letters of credit in favor of third parties	$ 209.4	$ 210.3
Undrawn outstanding letters of credit between subsidiaries	966.3	995.5
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	939.7	826.9
Insurance Treaty Guarantee Net Of Assets Held In Trust [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	753.0	647.9
Guarantee Of Borrowed Securities [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	184.5	93.0
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	880.1
Commitment Period Two Thousand Twenty Six [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	500.0	500.0
Commitment Period 2033 [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	1,350.0	1,350.0
Commitment Period Two Thousand Thirty Six [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	1,250.0	1,250.0
Limited Partner [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	302.5	239.5
Commercial Loan [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	23.5	4.6
Private Placement [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	39.8	22.0
Bank Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Unfunded commitments to fund partnership investments, mortgage loans and private corporate bond investments	$ 46.9	$ 37.0