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Investments Gross Unrealized losses by Major Security (Detail)
$ in Millions
Sep. 30, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Sep. 30, 2019
USD ($)
security
Number of Securities      
Total | security 214 307 243
Less than 12 Months | security 152 217 90
12 Months or Greater | security 62 90 153
Fair Value      
Total $ 4,934.0 $ 8,761.3 $ 4,812.7
Less than 12 Months 4,207.3 8,038.3 3,617.6
12 Months or Greater 726.7 723.0 1,195.1
Unrealized Losses      
Total (40.5) (57.6) (17.2)
Less than 12 Months (26.1) (53.8) (12.1)
12 Months or Greater $ (14.4) $ (3.8) $ (5.1)
U.S. government obligations      
Number of Securities      
Total | security 13 23 14
Less than 12 Months | security 13 19 7
12 Months or Greater | security 0 4 7
Fair Value      
Total $ 1,993.8 $ 5,152.4 $ 2,708.0
Less than 12 Months 1,993.8 5,057.2 2,534.4
12 Months or Greater 0.0 95.2 173.6
Unrealized Losses      
Total (1.6) (43.7) (9.2)
Less than 12 Months (1.6) (43.6) (8.9)
12 Months or Greater $ 0.0 $ (0.1) $ (0.3)
State and local government obligations      
Number of Securities      
Total | security 45 67 60
Less than 12 Months | security 45 52 25
12 Months or Greater | security 0 15 35
Fair Value      
Total $ 320.1 $ 314.3 $ 228.3
Less than 12 Months 320.1 287.5 100.4
12 Months or Greater 0.0 26.8 127.9
Unrealized Losses      
Total (0.5) (2.7) (1.1)
Less than 12 Months (0.5) (2.6) (0.7)
12 Months or Greater $ 0.0 $ (0.1) $ (0.4)
Corporate debt securities      
Number of Securities      
Total | security 28 16 24
Less than 12 Months | security 28 12 5
12 Months or Greater | security 0 4 19
Fair Value      
Total $ 537.8 $ 247.6 $ 245.5
Less than 12 Months 537.8 191.4 77.2
12 Months or Greater 0.0 56.2 168.3
Unrealized Losses      
Total (3.6) (0.5) (0.4)
Less than 12 Months (3.6) (0.5) (0.1)
12 Months or Greater $ 0.0 $ 0.0 $ (0.3)
Residential mortgage-backed securities      
Number of Securities      
Total | security 28 41 39
Less than 12 Months | security 9 12 5
12 Months or Greater | security 19 29 34
Fair Value      
Total $ 86.7 $ 292.8 $ 301.0
Less than 12 Months 38.7 163.7 138.7
12 Months or Greater 48.0 129.1 162.3
Unrealized Losses      
Total (1.4) (2.0) (1.5)
Less than 12 Months (1.0) (0.9) (0.1)
12 Months or Greater $ (0.4) $ (1.1) $ (1.4)
Commercial mortgage-backed securities      
Number of Securities      
Total | security 77 98 48
Less than 12 Months | security 46 79 26
12 Months or Greater | security 31 19 22
Fair Value      
Total $ 1,794.8 $ 1,742.4 $ 743.3
Less than 12 Months 1,170.4 1,400.0 361.9
12 Months or Greater 624.4 342.4 381.4
Unrealized Losses      
Total (27.9) (6.0) (1.9)
Less than 12 Months (17.3) (5.3) (0.8)
12 Months or Greater $ (10.6) $ (0.7) $ (1.1)
Other asset-backed securities      
Number of Securities      
Total | security 22 61 55
Less than 12 Months | security 11 43 20
12 Months or Greater | security 11 18 35
Fair Value      
Total $ 191.5 $ 1,000.6 $ 529.4
Less than 12 Months 146.5 938.5 359.1
12 Months or Greater 45.0 62.1 170.3
Unrealized Losses      
Total (2.3) (1.4) (0.6)
Less than 12 Months (2.1) (0.9) (0.2)
12 Months or Greater $ (0.2) $ (0.5) $ (0.4)
Redeemable preferred stocks      
Number of Securities      
Total | security 1 1 3
Less than 12 Months | security 0 0 2
12 Months or Greater | security 1 1 1
Fair Value      
Total $ 9.3 $ 11.2 $ 57.2
Less than 12 Months 0.0 0.0 45.9
12 Months or Greater 9.3 11.2 11.3
Unrealized Losses      
Total (3.2) (1.3) (2.5)
Less than 12 Months 0.0 0.0 (1.3)
12 Months or Greater $ (3.2) $ (1.3) $ (1.2)