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SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Deferred acquisition costs |  | $ 703.0 | $ 662.8 | $ 620.8
Unearned premiums |  | 3,440.4 | 3,283.3 | 3,102.5
Other policy claims and benefits payable |  | 15.4 | 14.1 | 12.8
Premium revenue |  | 6,161.1 | 5,912.6 | 5,663.1
Net investment income | [1] | 454.4 | 372.6 | 332.1
Benefits, claims, losses and settlement expenses |  | 3,729.3 | 3,757.4 | 4,134.6
Amortization of deferred acquisition costs |  | 1,285.2 | 1,221.7 | 1,176.0
Other operating expenses | [2] | 736.1 | 720.5 | 641.8
Premiums written |  | 6,322.1 | 6,083.6 | 5,810.2
Operating Segments [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Future policy benefits, losses, claims and loss expenses |  | 7,739.8 | 7,447.1 | 7,295.3
Interest On Debt [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Other operating expenses | [2] | 43.2 | 34.1 | 34.1
Eliminations [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Other operating expenses | [2] |  | (3.6) | (8.1)
Core Commercial [Member] | Operating Segments [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Deferred acquisition costs |  | 234.6 | 230.8 | 223.0
Future policy benefits, losses, claims and loss expenses |  | 3,853.1 | 3,606.3 | 3,461.5
Unearned premiums |  | 1,187.9 | 1,121.4 | 1,066.8
Other policy claims and benefits payable |  | 8.3 | 7.4 | 6.9
Premium revenue |  | 2,211.0 | 2,148.8 | 2,060.3
Net investment income | [1] | 202.5 | 170.4 | 151.8
Benefits, claims, losses and settlement expenses |  | 1,411.9 | 1,302.1 | 1,354.9
Amortization of deferred acquisition costs |  | 481.2 | 462.6 | 443.8
Other operating expenses | [2] | 274.7 | 277.9 | 250.9
Premiums written |  | 2,273.7 | 2,195.5 | 2,107.0
Specialty [Member] | Operating Segments [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Deferred acquisition costs |  | 227.0 | 209.2 | 193.6
Future policy benefits, losses, claims and loss expenses |  | 1,725.7 | 1,669.1 | 1,619.6
Unearned premiums |  | 850.8 | 800.4 | 755.1
Other policy claims and benefits payable |  | 7.1 | 6.7 | 5.9
Premium revenue |  | 1,398.3 | 1,322.0 | 1,274.2
Net investment income | [1] | 102.6 | 84.5 | 71.1
Benefits, claims, losses and settlement expenses |  | 676.9 | 646.7 | 645.5
Amortization of deferred acquisition costs |  | 367.2 | 347.4 | 329.5
Other operating expenses | [2] | 165.5 | 160.6 | 133.2
Premiums written |  | 1,441.5 | 1,373.9 | 1,293.3
Personal Lines [Member] | Operating Segments [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Deferred acquisition costs |  | 241.4 | 222.8 | 204.2
Future policy benefits, losses, claims and loss expenses |  | 2,131.4 | 2,108.8 | 2,148.8
Unearned premiums |  | 1,401.7 | 1,361.5 | 1,280.6
Premium revenue |  | 2,551.8 | 2,441.8 | 2,328.6
Net investment income | [1] | 128.5 | 106.7 | 96.8
Benefits, claims, losses and settlement expenses |  | 1,639.8 | 1,807.2 | 2,131.9
Amortization of deferred acquisition costs |  | 436.8 | 411.7 | 402.7
Other operating expenses | [2] | 238.6 | 234.0 | 211.6
Premiums written |  | 2,606.9 | 2,514.2 | 2,409.9
Other [Member] | Operating Segments [Member] |  |  |  |
Supplementary Insurance Information By Segment [Line Items] |  |  |  |
Future policy benefits, losses, claims and loss expenses |  | 29.6 | 62.9 | 65.4
Net investment income | [1] | 20.8 | 11.0 | 12.4
Benefits, claims, losses and settlement expenses |  | 0.7 | 1.4 | 2.3
Other operating expenses | [2] | $ 14.1 | $ 17.5 | $ 20.1
[1] | The Company manages investment assets for its Core Commercial, Specialty, Personal Lines and Other segments on a combined basis, based on the requirements of its combined insurance companies. Net investment income is allocated to these segments based on actuarial information related to the underlying businesses.
[2] | For other operating expenses that are not directly attributable to a single segment, expenses are allocated using a consistent and reasonable approach, generally based on net premiums earned.

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