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Loss and Loss Adjustment Expense Reserves (Tables)
 12 Months Ended
Dec. 31, 2025
Insurance [Abstract]  
Summary of loss and LAE reserve activity The following table summarizes the loss and LAE reserve activity of the Ark/WM Outrigger segment for the years ended December 31, 2025, 2024 and 2023:
Year Ended December 31,
Millions202520242023
Gross beginning balance$2,127.5 $1,605.1 $1,296.5 
Less: beginning reinsurance recoverable on unpaid losses (1)
(434.4)(340.8)(505.0)
Net loss and LAE reserves1,693.1 1,264.3 791.5 
Loss and LAE incurred relating to:
      Current year losses956.9 914.4 706.9 
      Prior year losses(112.7)(58.6)19.9 
Net incurred loss and LAE844.2 855.8 726.8 
Loss and LAE paid relating to:
Current year losses(205.6)(109.2)(51.3)
   Prior year losses(380.1)(307.5)(356.6)
Net paid loss and LAE(585.7)(416.7)(407.9)
Change in TPC Providers’ participation (2)
— — 145.4 
Foreign currency translation and other adjustments to loss and LAE reserves21.1 (10.3)8.5 
Net ending balance1,972.7 1,693.1 1,264.3 
Plus: ending reinsurance recoverable on unpaid losses
508.3 434.4 340.8 
Gross ending balance$2,481.0 $2,127.5 $1,605.1 
(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of January 1, 2023.
(2) Amount represents the impact to net loss and LAE reserves due to a change in the TPC Providers’ participation related to the annual RITC process.
The following table summarizes the unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses, for each of the Ark/WM Outrigger segment’s major reserving lines of business as of December 31, 2025 and 2024:

Year Ended December 31,


Millions		20252024
Property and Accident & Health (1)
$573.1 $533.6 
Marine & Energy569.0 463.1 
Specialty508.2 417.0 
Casualty-Active 230.6 178.0 
Casualty-Runoff57.1 66.5 
WM Outrigger Re34.7 34.9 
   Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses (1)
1,972.7 1,693.1 
Plus: Reinsurance recoverables on unpaid losses
Property and Accident & Health (2)
142.4 121.5 
Marine & Energy208.9 180.1 
Specialty44.4 51.3 
Casualty-Active113.0 80.7 
Casualty-Runoff(.4).8 
Total Reinsurance recoverables on unpaid losses (2)
508.3 434.4 
Total unpaid loss and LAE reserves$2,481.0 $2,127.5 
(1) The amounts exclude $5.2 and $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2025 and 2024, which eliminates in White Mountains’s consolidated financial statements.
(2) The amounts exclude $29.5 and $31.8 ceded by Ark to WM Outrigger Re as of December 31, 2025 and 2024, which eliminate in White Mountains’s consolidated financial statements.
Short-duration insurance contracts, claims development
Property and Accident & Health
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2025
Accident Year20162017201820192020202120222023
2024
2025
Total IBNR plus expected development on reported claims
Cumulative number of reported claims
Unaudited
2016$22.7 $17.8 $18.6 $18.7 $18.7 $18.9 $18.8 $18.8 $18.8 $18.8 $.2 3,434
201731.6 38.3 45.8 44.8 43.4 42.9 44.3 43.9 43.3 15.6 4,629
201841.4 47.8 $49.7 47.4 47.4 47.0 47.0 49.0 2.7 4,293
201934.6 $31.7 27.4 24.2 23.5 23.2 21.5 .5 4,030
202078.1 76.3 75.3 79.1 81.0 84.8 6.0 4,664
2021173.0 156.8 168.9 171.5 169.7 8.1 3,533
2022245.7 269.6 280.0 281.9 9.0 4,100
2023216.9 178.6 130.4 20.8 3,688
2024364.8 324.6 106.4 4,495
2025394.1 171.7 3,899
Total$1,518.1 
Property and Accident & Health
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2016201720182019202020212022202320242025
Unaudited
2016$8.6 $13.3 $16.6 $17.0 $17.1 $17.4 $18.0 $18.2 $18.4 $18.4 
201717.0 26.1 31.9 33.1 29.9 27.7 25.9 28.7 29.3 
201815.8 32.7 40.6 40.6 41.4 43.4 44.2 45.2 
20196.9 17.0 18.6 18.8 19.6 20.9 19.9 
202011.4 34.5 47.5 56.3 67.4 72.4 
202131.0 87.5 132.3 145.6 150.9 
202271.7 194.5 232.0 250.2 
202320.2 53.8 78.5 
202455.4 134.6 
2025142.8 
Total942.2 
All outstanding liabilities before 2016, net of reinsurance
2.4 
Loss and LAE reserves, net of reinsurance
$578.3 
Marine & Energy
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2025
Accident Year2016201720182019202020212022202320242025Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2016$23.8 $20.0 $16.0 $14.9 $14.6 $15.2 $14.5 $14.0 $13.7 $13.8 $— 3,776 
201726.5 19.7 17.9 17.3 16.9 16.0 16.3 16.4 16.4 .2 4,141 
201826.2 20.6 18.1 18.5 18.0 18.4 17.4 17.0 .2 3,245 
201924.2 22.0 22.1 21.9 22.3 21.6 21.8 .4 2,413 
202030.4 27.7 29.1 27.9 27.6 27.6 .8 1,594 
202187.7 70.5 68.4 75.6 79.8 3.8 1,530 
2022150.8 154.9 157.5 218.6 56.8 2,083 
2023198.6 189.9 123.2 36.8 2,341 
2024243.3 248.1 138.4 2,195 
2025235.3 184.9 1,618 
Total$1,001.6 
Marine & Energy
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year201620172018201920202021202220232024
2025
Unaudited
2016$5.6 $10.2 $12.9 $13.3 $13.4 $14.1 $13.8 $13.8 $13.8 $13.8 
20175.2 11.3 13.1 14.4 14.4 14.3 14.3 14.6 15.9 
20182.7 12.9 14.6 15.3 16.0 15.9 16.1 16.3 
20193.4 10.9 12.9 14.7 15.7 18.4 19.3 
20203.2 12.9 16.3 18.9 22.4 23.2 
20216.4 24.9 38.9 52.7 64.4 
202212.3 66.7 98.6 145.5 
202310.6 42.7 61.3 
202421.4 60.1 
202519.4 
Total439.2 
All outstanding liabilities before 2016, net of reinsurance
6.6 
Loss and LAE reserves, net of reinsurance
$569.0 
Specialty
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2025
Accident Year2016201720182019202020212022202320242025Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2016$18.5 $14.5 $11.1 $11.4 $12.0 $12.0 $9.1 $8.6 $12.5 $12.2 $.2 1,936
201718.3 13.2 12.3 11.8 11.9 10.9 10.6 11.2 11.1 — 2,201
201814.7 16.6 17.0 16.2 15.1 15.9 16.9 17.5 .9 2,129
201922.1 19.8 18.9 25.9 30.1 20.0 26.8 .7 2,396
202024.5 23.5 19.3 20.4 17.3 17.7 1.3 2,028
202171.8 63.0 51.8 44.0 41.7 6.6 1,735
2022181.9 178.5 172.6 179.6 58.3 1,577
2023216.6 199.4 153.4 41.3 1,734
2024223.2 231.3 133.2 1,798
2025226.8 169.4 1,831
Total$918.1 
Specialty
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2016201720182019202020212022202320242025
Unaudited
2016$3.2 $8.0 $9.2 $10.0 $10.4 $10.5 $8.7 $8.4 $11.8 $11.8 
20173.3 6.8 8.6 8.8 8.8 9.5 9.3 9.9 9.9 
20182.9 8.5 10.2 10.6 12.0 13.2 14.4 14.8 
20195.0 7.2 7.6 18.5 25.2 17.7 19.8 
20205.4 11.0 13.4 18.8 18.5 19.2 
20215.1 24.1 35.8 34.6 35.3 
202216.1 62.5 84.9 113.2 
202319.2 76.3 109.1 
202428.9 53.8 
202524.1 
Total411.0 
All outstanding liabilities before 2016, net of reinsurance
1.1 
Loss and LAE reserves, net of reinsurance
$508.2 
Casualty-Active
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2025
Accident Year2016201720182019202020212022202320242025Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2016$8.8 $8.3 $8.9 $9.0 $9.1 $9.2 $9.2 $10.1 $11.8 $11.9 $.4 1,606 
201711.6 11.7 10.8 9.3 9.1 10.5 10.7 10.9 10.6 .7 1,693 
201812.9 13.3 11.1 10.9 8.6 9.2 9.4 10.5 .9 1,175 
201914.8 13.7 12.3 10.6 11.4 13.0 14.4 1.4 1,062 
202013.5 12.1 10.9 9.2 8.8 9.5 1.7 731 
202121.4 22.4 16.6 16.3 21.7 4.1 1,050 
202233.0 38.1 34.7 17.3 9.8 1,774 
202361.0 65.9 57.1 39.4 2,234 
202460.0 82.0 71.9 2,171 
202568.2 63.4 1,723 
Total$303.2 
Casualty-Active
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2016201720182019202020212022202320242025
Unaudited
2016$.2 $1.0 $2.3 $4.0 $4.6 $5.3 $6.5 $8.1 $9.7 $10.3 
2017.8 1.7 2.8 3.4 4.2 5.7 7.5 8.3 9.1 
2018.3 1.4 3.5 4.3 4.3 6.2 7.1 8.0 
2019.3 1.4 2.3 3.0 5.7 8.3 10.2 
2020.5 1.0 2.0 3.3 5.3 6.0 
2021.5 .9 3.1 9.6 12.6 
2022.5 1.6 2.5 3.6 
2023.9 5.6 10.8 
20241.9 4.6 
20251.0 
Total76.2 
All outstanding liabilities before 2016, net of reinsurance
3.6 
Loss and LAE reserves, net of reinsurance
$230.6 
Casualty-Runoff
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2025
Accident Year
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2016$32.6 $32.3 $40.5 $38.6 $38.9 $38.6 $37.7 $37.5 $37.5 $38.1 $1.9 2,156 
201730.7 34.0 31.5 32.1 31.6 29.9 28.3 28.5 27.9 2.0 1,606 
201833.6 28.2 27.3 26.6 26.2 28.0 27.8 28.2 2.9 1,284 
201926.5 23.3 23.4 24.9 23.6 23.7 22.5 4.2 976 
202015.9 12.3 13.9 10.9 9.5 9.2 2.3 573 
202110.5 7.0 5.5 4.5 3.9 1.5 284 
2022.8 2.6 2.6 2.3 1.3 80 
20232.7 3.6 3.1 1.8 40 
20241.2 1.1 .5 20 
2025.7 — 41 
Total$137.0 
Casualty-Runoff
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2016201720182019202020212022202320242025
Unaudited
2016$3.9 $10.2 $17.7 $22.7 $25.4 $27.8 $28.8 $31.0 $32.5 $34.1 
20173.2 9.4 14.7 18.5 21.4 22.5 22.8 23.5 24.8 
20183.4 7.4 12.6 14.9 16.3 18.2 21.3 22.7 
20193.3 5.8 7.8 12.1 15.1 15.9 17.3 
2020.8 1.3 3.1 6.0 6.3 6.5 
2021.5 1.7 1.9 2.3 2.4 
2022.3 .6 .7 .9 
2023.9 1.0 1.1 
2024.5 .5 
2025.6 
Total110.9 
All outstanding liabilities before 2016, net of reinsurance
31.0 
Loss and LAE reserves, net of reinsurance
$57.1 
Short-duration insurance contracts, schedule of historical claims duration
Property and Accident & Health
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
27.4%31.9%18.4%6.4%2.9%1.9%0.4%0.9%0.2%0.1%
Marine & Energy
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
12.1%27.1%17.3%12.1%6.4%6.5%0.5%0.4%0.1%0.1%
Specialty
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
18.2%26.8%13.2%8.7%6.3%2.5%2.1%1.8%(1.2)%1.0%
Casualty-Active
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
3.8%7.5%11.4%12.9%10.0%11.2%7.2%5.1%4.2%3.4%
Casualty-Runoff
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
8.8%13.9%15.3%14.8%8.5%6.7%5.9%4.2%3.2%2.2%