Commitments and Contingent Liabilities (Narrative Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commitments And Contingent Liabilities [Line Items]
Undrawn Outstanding Letters Of Credit In Favor Of Third Parties	$ 45,400,000	$ 15,800,000
Undrawn Outstanding Letters Of Credit Between Subsidiaries	763,500,000	582,900,000
Syndicated Revolving Credit Facility Capacity	850,000,000
Syndicated Revolving Credit Facility Capacity Undrawn Outstanding	402,900,000	183,500,000
Operating Leases, Rent Expense	19,500,000	18,800,000	17,100,000
Syndicated Credit Facility $850 Million [Member]
Commitments And Contingent Liabilities [Line Items]
Syndicated Revolving Credit Facility Capacity	850,000,000
Insurance Treaty Guarantee [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	686,000,000	697,500,000
Guarantee Of Borrowed Securities [Member]
Commitments And Contingent Liabilities [Line Items]
Guarantor Obligations, Current Carrying Value	87,500,000	150,000,000
Guarantee Of Lease Obligations [Member]
Commitments And Contingent Liabilities [Line Items]
Operating Leases, Rent Expense	10,100,000	11,700,000
Guarantee Of Subsidiary Collateral Financing [Member]
Commitments And Contingent Liabilities [Line Items]
Operating Leases, Rent Expense	300,000,000
Limited Partnerships [Member]
Commitments And Contingent Liabilities [Line Items]
Commitments To Extend Credit	176,700,000	156,600,000
Commercial Mortgage Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Commitments To Extend Credit	22,200,000	33,600,000
Private Placement Investments [Member]
Commitments And Contingent Liabilities [Line Items]
Commitments To Extend Credit		100,000,000
Bank Loans [Member]
Commitments And Contingent Liabilities [Line Items]
Commitments To Extend Credit	$ 68,500,000