Fair Value (Financial Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 1,205.0	$ 1,606.0
Total debt securities	9,453.0	8,373.0
Securities lending invested collateral	0	50.0
Total invested assets	10,658.0	10,029.0
Debt Securities [Member] | Tax-Exempt Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	2,856.0	2,433.0
Debt Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	44.0	56.0
Debt Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	381.0	321.0
Debt Securities [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	83.0	150.0
Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	3,580.0	3,032.0
Debt Securities [Member] | Redeemable Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	5.0
Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | U.S. Treasury And Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	725.0	712.0
Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	1,784.0	1,664.0
Quoted Prices In Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,205.0	1,606.0
Total debt securities	0	0
Securities lending invested collateral	0	25.0
Total invested assets	1,205.0	1,631.0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Tax-Exempt Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | Redeemable Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | U.S. Treasury And Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Quoted Prices In Active Markets (Level 1) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Total debt securities	9,412.0	8,307.0
Securities lending invested collateral	0	25.0
Total invested assets	9,412.0	8,332.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Tax-Exempt Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	2,840.0	2,381.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	44.0	56.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	381.0	321.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	82.0	148.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	3,556.0	3,025.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Redeemable Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | U.S. Treasury And Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	725.0	712.0
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	1,784.0	1,664.0
Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Total debt securities	41.0	66.0
Securities lending invested collateral	0	0
Total invested assets	41.0	66.0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Tax-Exempt Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	16.0	52.0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	1.0	2.0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	24.0	7.0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | Redeemable Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	5.0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | U.S. Treasury And Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	0	0
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities	$ 0	$ 0