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Investments Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales $ 148.0 $ 291.6
Equity Securities, FV-NI, Gross Realized Gains on Sales 18.4 85.6
Debt and Equity Securities, Gross Realized Gains on Sales 166.4 377.2
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (22.6) (3.7)
Equity Securities, FV-NI, Gross Realized Losses on Sales 0.0 (58.3)
Debt and Equity Securities, Gross Realized Losses on Sales (22.6) (62.0)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 125.4 287.9
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales 18.4 27.3
Gross Realized Gain (Loss), excluding Holding Period and OTTI 143.8 315.2
Unrealized Gain (Loss) on Hybrid Instrument, Net (10.9) (31.2)
Net holding period gains (losses), equity securities 452.4 (837.6)
Net Holding Period Realized Gain (Loss) 441.5 (868.8)
Total net realized gains (losses) on securities 585.3 (553.6)
U.S. government obligations    
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales 66.6 251.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (19.6) (2.7)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 47.0 248.4
State and local government obligations    
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales 30.2 2.0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (0.2) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 30.0 2.0
Corporate debt securities    
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales 20.7 32.4
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (2.1) (1.0)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 18.6 31.4
Commercial mortgage-backed securities    
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales 29.8 6.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (0.6) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 29.2 6.1
Other asset-backed securities    
Gain (Loss) on Securities [Line Items]    
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales 0.7 0.0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (0.1) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment 0.6 0.0
Nonredeemable preferred stocks    
Gain (Loss) on Securities [Line Items]    
Equity Securities, FV-NI, Gross Realized Gains on Sales 17.3 19.6
Equity Securities, FV-NI, Gross Realized Losses on Sales 0.0 (4.4)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales 17.3 15.2
Common equities    
Gain (Loss) on Securities [Line Items]    
Equity Securities, FV-NI, Gross Realized Gains on Sales 1.1 66.0
Equity Securities, FV-NI, Gross Realized Losses on Sales 0.0 (53.9)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales $ 1.1 $ 12.1