v2.4.0.6
Fair Value Measurements - Fair Value Inputs (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading Securities, Fair Value Disclosure $ 4.6 $ 4.4
Investments, Fair Value Disclosure 5,221.9 5,175.1
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Fixed Maturities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 4,787.7 4,773.4
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | US Government and Government Agencies and Authorities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 474.4 491.7
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | States and Political Subdivisions [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 1,825.4 1,852.6
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Bonds and Notes [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 2,401.5 2,342.3
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Redeemable Preferred Stocks [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 81.4 81.7
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Mortgage and Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 5.0 5.1
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Equity Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 429.6 397.3
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Preferred Stock Finance, Insurance and Real Estate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 97.4 86.7
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Preferred Stocks Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 21.2 20.5
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Common Stocks Manufacturing [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 87.1 83.4
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Common Stock Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 49.7 47.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Exchange Traded Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 67.7 66.6
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Limited Liability Companies and Limited Partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 106.5 93.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading Securities, Fair Value Disclosure 4.6 4.4
Investments, Fair Value Disclosure 330.7 342.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 133.1 153.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government and Government Agencies and Authorities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 133.1 153.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | States and Political Subdivisions [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Bonds and Notes [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Redeemable Preferred Stocks [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Mortgage and Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 193.0 184.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Stock Finance, Insurance and Real Estate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Stocks Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stocks Manufacturing [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 80.2 74.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 45.1 42.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Exchange Traded Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 67.7 66.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Limited Liability Companies and Limited Partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading Securities, Fair Value Disclosure 0 0
Investments, Fair Value Disclosure 4,506.6 4,484.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 4,388.8 4,378.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Government and Government Agencies and Authorities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 341.3 338.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 1,825.4 1,852.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Bonds and Notes [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 2,142.3 2,107.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Redeemable Preferred Stocks [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 75.1 75.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage and Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 4.7 4.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 117.8 106.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Preferred Stock Finance, Insurance and Real Estate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 97.4 86.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Preferred Stocks Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 15.2 14.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stocks Manufacturing [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 5.2 5.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stock Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Exchange Traded Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Limited Liability Companies and Limited Partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading Securities, Fair Value Disclosure 0 0
Investments, Fair Value Disclosure 384.6 348.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 265.8 241.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Government and Government Agencies and Authorities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | States and Political Subdivisions [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Bonds and Notes [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 259.2 235.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Redeemable Preferred Stocks [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 6.3 6.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage and Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0.3 0.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 118.8 106.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Stock Finance, Insurance and Real Estate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Stocks Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 6.0 5.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stocks Manufacturing [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 1.7 3.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stock Other Industries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 4.6 4.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Exchange Traded Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Limited Liability Companies and Limited Partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Fair Value Disclosure 106.5 93.1
Private Equity Funds and Mezzanine Debt Funds [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments $ 90.6