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Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions | Jun. 30, 2024 USD ($) security | Dec. 31, 2023 USD ($) security | Jun. 30, 2023 USD ($) security
--- | --- | --- | ---
Number of Securities |  |  |
Total | security | 1,257 | 1,223 | 1,479
Less than 12 Months | security | 301 | 126 | 306
12 Months or Greater | security | 956 | 1,097 | 1,173
Fair Value |  |  |
Total | $ 48,226.7 | $ 44,062.1 | $ 51,363.7
Less than 12 Months | 18,659.5 | 13,347.6 | 22,029.5
12 Months or Greater | 29,567.2 | 30,714.5 | 29,334.2
Unrealized Losses |  |  |
Total | (2,267.4) | (2,346.8) | (3,367.6)
Less than 12 Months | (135.0) | (119.3) | (496.9)
12 Months or Greater | $ (2,132.4) | $ (2,227.5) | $ (2,870.7)
U.S. government obligations |  |  |
Number of Securities |  |  |
Total | security | 117 | 147 | 171
Less than 12 Months | security | 17 | 25 | 51
12 Months or Greater | security | 100 | 122 | 120
Fair Value |  |  |
Total | $ 29,776.3 | $ 28,225.0 | $ 31,496.1
Less than 12 Months | 12,936.0 | 11,890.0 | 17,704.6
12 Months or Greater | 16,840.3 | 16,335.0 | 13,791.5
Unrealized Losses |  |  |
Total | (1,228.7) | (1,157.9) | (1,676.7)
Less than 12 Months | (109.4) | (100.0) | (427.9)
12 Months or Greater | $ (1,119.3) | $ (1,057.9) | $ (1,248.8)
State and local government obligations |  |  |
Number of Securities |  |  |
Total | security | 336 | 324 | 346
Less than 12 Months | security | 58 | 31 | 57
12 Months or Greater | security | 278 | 293 | 289
Fair Value |  |  |
Total | $ 2,078.5 | $ 1,846.2 | $ 1,998.2
Less than 12 Months | 410.9 | 169.9 | 346.8
12 Months or Greater | 1,667.6 | 1,676.3 | 1,651.4
Unrealized Losses |  |  |
Total | (144.3) | (138.4) | (181.9)
Less than 12 Months | (2.0) | (0.9) | (5.2)
12 Months or Greater | $ (142.3) | $ (137.5) | $ (176.7)
Foreign government obligations |  |  |
Number of Securities |  |  |
Total | security | 1 | 1 | 1
Less than 12 Months | security | 0 | 0 | 0
12 Months or Greater | security | 1 | 1 | 1
Fair Value |  |  |
Total | $ 15.8 | $ 16.3 | $ 15.8
Less than 12 Months | 0.0 | 0.0 | 0.0
12 Months or Greater | 15.8 | 16.3 | 15.8
Unrealized Losses |  |  |
Total | (0.9) | (1.0) | (1.4)
Less than 12 Months | 0.0 | 0.0 | 0.0
12 Months or Greater | $ (0.9) | $ (1.0) | $ (1.4)
Corporate debt securities |  |  |
Number of Securities |  |  |
Total | security | 402 | 313 | 414
Less than 12 Months | security | 150 | 26 | 109
12 Months or Greater | security | 252 | 287 | 305
Fair Value |  |  |
Total | $ 8,995.0 | $ 6,642.4 | $ 8,565.9
Less than 12 Months | 3,307.1 | 617.2 | 2,397.4
12 Months or Greater | 5,687.9 | 6,025.2 | 6,168.5
Unrealized Losses |  |  |
Total | (303.8) | (332.3) | (540.3)
Less than 12 Months | (18.4) | (14.7) | (56.0)
12 Months or Greater | $ (285.4) | $ (317.6) | $ (484.3)
Residential mortgage-backed securities |  |  |
Number of Securities |  |  |
Total | security | 39 | 39 | 41
Less than 12 Months | security | 7 | 2 | 7
12 Months or Greater | security | 32 | 37 | 34
Fair Value |  |  |
Total | $ 308.0 | $ 88.4 | $ 108.5
Less than 12 Months | 244.1 | 0.4 | 2.4
12 Months or Greater | 63.9 | 88.0 | 106.1
Unrealized Losses |  |  |
Total | (9.4) | (10.0) | (13.0)
Less than 12 Months | (0.5) | 0.0 | (0.1)
12 Months or Greater | $ (8.9) | $ (10.0) | $ (12.9)
Commercial mortgage-backed securities |  |  |
Number of Securities |  |  |
Total | security | 183 | 189 | 211
Less than 12 Months | security | 7 | 1 | 4
12 Months or Greater | security | 176 | 188 | 207
Fair Value |  |  |
Total | $ 3,883.2 | $ 3,912.2 | $ 4,247.9
Less than 12 Months | 314.4 | 30.7 | 13.6
12 Months or Greater | 3,568.8 | 3,881.5 | 4,234.3
Unrealized Losses |  |  |
Total | (489.1) | (597.7) | (718.7)
Less than 12 Months | (1.3) | (2.5) | (0.1)
12 Months or Greater | $ (487.8) | $ (595.2) | $ (718.6)
Other asset-backed securities |  |  |
Number of Securities |  |  |
Total | security | 176 | 207 | 292
Less than 12 Months | security | 62 | 41 | 78
12 Months or Greater | security | 114 | 166 | 214
Fair Value |  |  |
Total | $ 3,133.3 | $ 3,299.1 | $ 4,900.0
Less than 12 Months | 1,447.0 | 639.4 | 1,564.7
12 Months or Greater | 1,686.3 | 2,659.7 | 3,335.3
Unrealized Losses |  |  |
Total | (89.3) | (107.1) | (232.0)
Less than 12 Months | (3.4) | (1.2) | (7.6)
12 Months or Greater | $ (85.9) | $ (105.9) | $ (224.4)
Redeemable preferred stocks |  |  |
Number of Securities |  |  |
Total | security | 3 | 3 | 3
Less than 12 Months | security | 0 | 0 | 0
12 Months or Greater | security | 3 | 3 | 3
Fair Value |  |  |
Total | $ 36.6 | $ 32.5 | $ 31.3
Less than 12 Months | 0.0 | 0.0 | 0.0
12 Months or Greater | 36.6 | 32.5 | 31.3
Unrealized Losses |  |  |
Total | (1.9) | (2.4) | (3.6)
Less than 12 Months | 0.0 | 0.0 | 0.0
12 Months or Greater | $ (1.9) | $ (2.4) | $ (3.6)

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