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Investments Investment Portfolio by Major Security Type (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 25,276.9 $ 22,031.7
Gross Unrealized Gains 2,097.0 1,588.9
Gross Unrealized Losses (99.4) (139.8)
Net Realized Gains (Losses) 0.2 1.8
Fair Value $ 27,274.7 $ 23,482.6
% of Total Fair Value 100.00% 100.00%
Other Receivables from Broker-Dealers and Clearing Organizations $ 5.8  
Other Payables to Broker-Dealers and Clearing Organizations   $ 27.8
Securities Held In Consolidated Non-Insurance Subsidiary 1,600.0 1,300.0
Fixed maturities    
Schedule of Available-for-sale Securities [Line Items]    
Cost 20,209.9 16,287.1
Gross Unrealized Gains 82.0 76.5
Gross Unrealized Losses (90.4) (121.6)
Net Realized Gains (Losses) 0.2 1.8
Fair Value $ 20,201.7 $ 16,243.8
% of Total Fair Value 74.10% 69.20%
Fixed maturities | U.S. government obligations    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 6,688.8 $ 2,899.2
Gross Unrealized Gains 1.1 0.0
Gross Unrealized Losses (44.0) (29.1)
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 6,645.9 $ 2,870.1
% of Total Fair Value 24.40% 12.20%
Fixed maturities | State and local government obligations    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 2,285.6 $ 2,509.5
Gross Unrealized Gains 20.7 13.8
Gross Unrealized Losses (9.3) (20.7)
Net Realized Gains (Losses) 0.1 0.0
Fair Value $ 2,297.1 $ 2,502.6
% of Total Fair Value 8.40% 10.70%
Fixed maturities | Foreign government obligations    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 0.0 $ 24.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 0.0 $ 24.5
% of Total Fair Value 0.00% 0.10%
Fixed maturities | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 4,997.2 $ 4,557.8
Gross Unrealized Gains 14.8 17.3
Gross Unrealized Losses (14.4) (24.3)
Net Realized Gains (Losses) 0.1 0.1
Fair Value $ 4,997.7 $ 4,550.9
% of Total Fair Value 18.30% 19.40%
Fixed maturities | Residential mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 828.8 $ 1,489.7
Gross Unrealized Gains 11.3 23.7
Gross Unrealized Losses (3.4) (15.6)
Net Realized Gains (Losses) 0.0 1.5
Fair Value $ 836.7 $ 1,499.3
% of Total Fair Value 3.10% 6.40%
Fixed maturities | Commercial mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 2,760.1 $ 2,266.9
Gross Unrealized Gains 11.8 12.0
Gross Unrealized Losses (13.3) (25.5)
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 2,758.6 $ 2,253.4
% of Total Fair Value 10.10% 9.60%
Fixed maturities | Other asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 2,454.5 $ 2,350.7
Gross Unrealized Gains 4.5 4.6
Gross Unrealized Losses (4.5) (4.4)
Net Realized Gains (Losses) 0.2 0.2
Fair Value $ 2,454.7 $ 2,351.1
% of Total Fair Value 9.00% 10.00%
Fixed maturities | Redeemable preferred stocks    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 194.9 $ 188.8
Gross Unrealized Gains 17.8 5.1
Gross Unrealized Losses (1.5) (2.0)
Net Realized Gains (Losses) (0.2) 0.0
Fair Value $ 211.0 $ 191.9
% of Total Fair Value 0.80% 0.80%
Equity securities | Nonredeemable preferred stocks    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 698.6 $ 734.2
Gross Unrealized Gains 114.0 135.4
Gross Unrealized Losses (8.8) (16.1)
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 803.8 $ 853.5
% of Total Fair Value 2.90% 3.60%
Equity securities | Common equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 1,499.0 $ 1,437.5
Gross Unrealized Gains 1,901.0 1,377.0
Gross Unrealized Losses (0.2) (2.1)
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 3,399.8 $ 2,812.4
% of Total Fair Value 12.50% 12.00%
Short-term investments    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 2,869.4 $ 3,572.9
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Net Realized Gains (Losses) 0.0 0.0
Fair Value $ 2,869.4 $ 3,572.9
% of Total Fair Value 10.50% 15.20%