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Investments - Gross Unrealized losses by Major Security (Detail)
$ in Millions
Sep. 30, 2023
USD ($)
security
Dec. 31, 2022
USD ($)
security
Sep. 30, 2022
USD ($)
security
Number of Securities      
Total | security 1,531 1,468 1,522
Less than 12 Months | security 353 873 1,223
12 Months or Greater | security 1,178 595 299
Fair Value      
Total $ 54,926.2 $ 44,315.9 $ 43,030.1
Less than 12 Months 22,483.5 28,119.4 32,956.5
12 Months or Greater 32,442.7 16,196.5 10,073.6
Unrealized Losses      
Total (3,846.3) (3,551.5) (3,943.1)
Less than 12 Months (668.1) (1,505.0) (2,550.4)
12 Months or Greater $ (3,178.2) $ (2,046.5) $ (1,392.7)
U.S. government obligations      
Number of Securities      
Total | security 173 160 157
Less than 12 Months | security 37 90 103
12 Months or Greater | security 136 70 54
Fair Value      
Total $ 34,293.8 $ 24,802.5 $ 22,385.4
Less than 12 Months 16,686.2 17,327.2 16,085.8
12 Months or Greater 17,607.6 7,475.3 6,299.6
Unrealized Losses      
Total (2,098.4) (1,604.7) (1,855.4)
Less than 12 Months (557.6) (699.2) (1,095.6)
12 Months or Greater $ (1,540.8) $ (905.5) $ (759.8)
State and local government obligations      
Number of Securities      
Total | security 356 348 353
Less than 12 Months | security 46 239 292
12 Months or Greater | security 310 109 61
Fair Value      
Total $ 2,033.9 $ 1,948.8 $ 1,911.0
Less than 12 Months 359.6 1,124.2 1,363.1
12 Months or Greater 1,674.3 824.6 547.9
Unrealized Losses      
Total (203.3) (202.9) (215.4)
Less than 12 Months (7.6) (76.8) (129.8)
12 Months or Greater $ (195.7) $ (126.1) $ (85.6)
Foreign government obligations      
Number of Securities      
Total | security 1 1 1
Less than 12 Months | security 0 0 1
12 Months or Greater | security 1 1 0
Fair Value      
Total $ 15.4 $ 15.5 $ 15.3
Less than 12 Months 0.0 0.0 15.3
12 Months or Greater 15.4 15.5 0.0
Unrealized Losses      
Total (1.5) (1.3) (1.4)
Less than 12 Months 0.0 0.0 (1.4)
12 Months or Greater $ (1.5) $ (1.3) $ 0.0
Corporate debt securities      
Number of Securities      
Total | security 467 422 452
Less than 12 Months | security 164 285 391
12 Months or Greater | security 303 137 61
Fair Value      
Total $ 9,490.0 $ 8,449.6 $ 8,885.6
Less than 12 Months 3,380.4 5,717.6 7,875.5
12 Months or Greater 6,109.6 2,732.0 1,010.1
Unrealized Losses      
Total (591.8) (676.1) (832.5)
Less than 12 Months (94.1) (426.1) (699.0)
12 Months or Greater $ (497.7) $ (250.0) $ (133.5)
Residential mortgage-backed securities      
Number of Securities      
Total | security 40 45 44
Less than 12 Months | security 2 27 34
12 Months or Greater | security 38 18 10
Fair Value      
Total $ 99.9 $ 151.0 $ 165.1
Less than 12 Months 0.4 65.1 126.0
12 Months or Greater 99.5 85.9 39.1
Unrealized Losses      
Total (13.0) (17.5) (16.5)
Less than 12 Months 0.0 (6.8) (11.8)
12 Months or Greater $ (13.0) $ (10.7) $ (4.7)
Commercial mortgage-backed securities      
Number of Securities      
Total | security 196 226 234
Less than 12 Months | security 0 99 182
12 Months or Greater | security 196 127 52
Fair Value      
Total $ 3,960.8 $ 4,651.1 $ 5,074.3
Less than 12 Months 0.0 1,702.0 3,856.0
12 Months or Greater 3,960.8 2,949.1 1,218.3
Unrealized Losses      
Total (723.9) (784.0) (747.4)
Less than 12 Months 0.0 (192.1) (425.4)
12 Months or Greater $ (723.9) $ (591.9) $ (322.0)
Other asset-backed securities      
Number of Securities      
Total | security 295 262 277
Less than 12 Months | security 104 130 217
12 Months or Greater | security 191 132 60
Fair Value      
Total $ 5,001.8 $ 4,247.8 $ 4,544.8
Less than 12 Months 2,056.9 2,144.8 3,597.1
12 Months or Greater 2,944.9 2,103.0 947.7
Unrealized Losses      
Total (210.1) (260.5) (268.9)
Less than 12 Months (8.8) (100.9) (183.4)
12 Months or Greater $ (201.3) $ (159.6) $ (85.5)
Redeemable preferred stocks      
Number of Securities      
Total | security 3 4 4
Less than 12 Months | security 0 3 3
12 Months or Greater | security 3 1 1
Fair Value      
Total $ 30.6 $ 49.6 $ 48.6
Less than 12 Months 0.0 38.5 37.7
12 Months or Greater 30.6 11.1 10.9
Unrealized Losses      
Total (4.3) (4.5) (5.6)
Less than 12 Months 0.0 (3.1) (4.0)
12 Months or Greater $ (4.3) $ (1.4) $ (1.6)