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Fair Value of Financial Instruments - Quantitative Estimates for Level 3 Assets (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2013
US Treasury and Government | Level 3
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
U.S. government and agency $ 62.0 $ 58.8
State and political subdivision | Level 3
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
State and political subdivision 50.1 45.4
Corporate | Level 3
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Corporate 954.9 874.1
Other asset-backed (“ABS”) | Level 3
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Other ABS 10.1 11.3
Fair value investments | Level 3
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair value investments $ 1.0 $ 0.8
Market Approach Valuation Technique | US Treasury and Government | Minimum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 0.89% 1.07%
Market Approach Valuation Technique | US Treasury and Government | Maximum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 3.95% 5.00%
Market Approach Valuation Technique | US Treasury and Government | Weighted Average
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 3.01% 3.18%
Market Approach Valuation Technique | State and political subdivision | Minimum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 2.34% 2.44%
Market Approach Valuation Technique | State and political subdivision | Maximum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 4.80% 5.79%
Market Approach Valuation Technique | State and political subdivision | Weighted Average
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 3.66% 3.88%
Market Approach Valuation Technique | Corporate | Minimum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 0.85% 1.06%
Market Approach Valuation Technique | Corporate | Maximum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 6.93% 6.75%
Market Approach Valuation Technique | Corporate | Weighted Average
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 3.45% 3.81%
Income Approach Valuation Technique | Other asset-backed (“ABS”)
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Prepayment rate   2.00%
Default rate   2.53%
Default rate term 2   0.37%
Recovery rate   10.00%
Income Approach Valuation Technique | Other asset-backed (“ABS”) | Minimum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 1.75% 2.10%
Income Approach Valuation Technique | Other asset-backed (“ABS”) | Maximum
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 3.15% 3.41%
Income Approach Valuation Technique | Other asset-backed (“ABS”) | Weighted Average
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Yield 1.97% 2.30%
Income Approach Valuation Technique | Fair value investments
    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Default rate 0.15% 0.25%
Recovery rate 42.00% 45.00%