v2.4.0.6
Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period $ 348.1 $ 241.0
Included in Condensed Consolidated Statements of Income 3.6 1.5
Included in Other Comprehensive Income 1.0 (2.5)
Purchases 61.3 40.2
Settlements (20.2) (16.9)
Sales (4.0) (2.9)
Transfers into Level 3 from Level 2 0.9 0
Transfers out of Level 3 into Level 2 (6.1) 0
Balance at End of Period 384.6 260.4
Fair Value, Level 1 to level 2 Transfers, Amount 0 0
Fair Value, Level 2 to level 1 Transfers, Amount 0 0
Fair Value, Level 1 to level 3 Transfers, Amount 0  
Fair Value, Level 3 to level 1 Transfers, Amount 0  
Corporate Bonds and Notes [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period 235.1 146.2
Included in Condensed Consolidated Statements of Income 0.1 0
Included in Other Comprehensive Income 0.9 1.2
Purchases 44.8 32.4
Settlements (16.2) (11.1)
Sales (0.3) (0.5)
Transfers into Level 3 from Level 2 0.9  
Transfers out of Level 3 into Level 2 (6.1)  
Balance at End of Period 259.2 168.2
Redeemable Preferred Stocks [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period 6.1 4.5
Included in Condensed Consolidated Statements of Income 0 0
Included in Other Comprehensive Income 0.2 0.9
Purchases 0 0
Settlements 0 0
Sales 0 0
Transfers into Level 3 from Level 2 0  
Transfers out of Level 3 into Level 2 0  
Balance at End of Period 6.3 5.4
Mortgage and Asset-backed [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period 0.3 0.4
Included in Condensed Consolidated Statements of Income 0 0
Included in Other Comprehensive Income 0 0
Purchases 0 0
Settlements 0 0
Sales 0 0
Transfers into Level 3 from Level 2 0  
Transfers out of Level 3 into Level 2 0  
Balance at End of Period 0.3 0.4
Preferred and Common Stocks [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period 13.5 14.7
Included in Condensed Consolidated Statements of Income 3.2 1.5
Included in Other Comprehensive Income (1.2) (2.3)
Purchases 0.5 0.3
Settlements 0 0
Sales (3.7) (2.4)
Transfers into Level 3 from Level 2 0  
Transfers out of Level 3 into Level 2 0  
Balance at End of Period 12.3 11.8
Other Equity Interests [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at Beginning of Period 93.1 75.2
Included in Condensed Consolidated Statements of Income 0.3 0
Included in Other Comprehensive Income 1.1 (2.3)
Purchases 16.0 7.5
Settlements (4.0) (5.8)
Sales 0 0
Transfers into Level 3 from Level 2 0  
Transfers out of Level 3 into Level 2 0  
Balance at End of Period $ 106.5 $ 74.6