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Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
security
$ / shares
Dec. 31, 2016
USD ($)
security
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value $ 27,274.7 $ 23,482.6
Equity securities | Nonredeemable preferred stocks    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 803.8 853.5
Fixed maturities    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 20,201.7 16,243.8
Fixed maturities | Commercial mortgage-backed securities    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 2,758.6 2,253.4
Fair Value, Inputs, Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 5.3 0.7
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 0.3 0.4
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value $ 5.0 $ 0.3
Fair Value, Inputs, Level 3 | Internal Pricing    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Number of securities | security 1 0
Fair Value $ 5.0  
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 5.0 $ 0.0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Internal Pricing    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value $ 5.0 0.0
Valuation Technique Internal price  
Fair Value Inputs, Original Issue Price | $ / shares $ 3.9  
Fair Value, Inputs, Level 3 | Fixed maturities    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value $ 0.0 0.3
Fair Value, Inputs, Level 3 | Fixed maturities | External Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value 0.0 $ 0.3
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | External Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Number of securities | security   1
Fair Value $ 0.0 $ 0.3
Fair Value Inputs, Prepayment Rate   0.00%
Valuation Technique   External vendor