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Loss and Loss Adjustment Expense Reserves (Tables)
 12 Months Ended
Dec. 31, 2024
Insurance Loss Reserves [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, 2024, 2023 and 2022:
Year Ended December 31,
Millions202420232022
Gross beginning balance$1,605.1 $1,296.5 $894.7 
Less: beginning reinsurance recoverable on unpaid losses (1)
(340.8)(505.0)(428.9)
Net loss and LAE reserves1,264.3 791.5 465.8 
Loss and LAE incurred relating to:
      Current year losses914.4 706.9 588.1 
      Prior year losses(58.6)19.9 (51.7)
Net incurred loss and LAE855.8 726.8 536.4 
Loss and LAE paid relating to:
Current year losses(109.2)(51.3)(98.9)
   Prior year losses(307.5)(356.6)(158.6)
Net paid loss and LAE(416.7)(407.9)(257.5)
Change in TPC Providers’ participation (2)
— 145.4 57.5 
Foreign currency translation and other adjustments to loss and LAE reserves(10.3)8.5 (10.7)
Net ending balance1,693.1 1,264.3 791.5 
Plus: ending reinsurance recoverable on unpaid losses (3)
434.4 340.8 505.0 
Gross ending balance$2,127.5 $1,605.1 $1,296.5 
(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 and $276.8 as of January 1, 2023 and 2022.
(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.
(3) The ending reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4 as of December 31, 2022.
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, 2024 and 2023:

Year Ended December 31,


Millions		20242023
Property and Accident & Health (1)
$533.6 $358.7 
Marine & Energy463.1 331.7 
Specialty417.0 339.7 
Casualty-Active 178.0 137.1 
Casualty-Runoff66.5 81.3 
WM Outrigger Re34.9 15.8 
   Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses (1)
1,693.1 1,264.3 
Plus: Reinsurance recoverables on unpaid losses
Property and Accident & Health (2)
121.5 126.9 
Marine & Energy180.1 59.1 
Specialty51.3 77.6 
Casualty-Active80.7 74.8 
Casualty-Runoff.8 2.4 
Total Reinsurance recoverables on unpaid losses (2)
434.4 340.8 
Total unpaid loss and LAE reserves$2,127.5 $1,605.1 
(1) The amounts exclude $3.1 ceded by Ark to WM Outrigger Re as of December 31, 2024, which eliminates in White Mountains’s consolidated financial statements.
(2) The amounts exclude $31.8 and $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2024 and 2023, 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, 2024
Accident Year2015201620172018201920202021202220232024
Total IBNR plus expected development on reported claims
Cumulative number of reported claims
Unaudited
2015$19.1 $18.1 $17.2 $16.2 $16.0 $16.0 $15.8 $15.7 $16.0 $16.1 $.1 2,829
201622.2 17.5 18.2 18.4 18.3 18.5 18.5 18.5 18.5 .2 3,433
201731.1 37.7 $45.2 44.2 42.8 42.3 43.8 43.4 15.8 4,624
201840.7 $47.1 49.0 46.7 46.8 46.3 46.4 1.9 4,288
201933.9 31.2 27.0 23.8 23.2 22.8 .6 4,024
202076.9 75.1 74.5 77.6 79.5 9.9 4,646
2021170.0 153.7 165.3 168.0 7.5 3,509
2022241.5 266.9 277.3 12.9 4,044
2023213.9 176.1 77.3 3,598
2024359.5 202.7 3,614
Total$1,207.6 
Property and Accident & Health
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2015201620172018201920202021202220232024
Unaudited
2015$6.9 $12.2 $13.4 $14.6 $14.5 $14.8 $15.0 $15.0 $15.4 $15.6 
20168.5 13.0 16.3 16.7 16.8 17.1 17.7 17.9 18.1 
201716.8 25.7 31.5 32.7 29.4 27.1 25.4 28.2 
201815.6 32.2 40.1 40.0 40.8 42.8 43.6 
20196.8 16.7 18.3 18.5 19.3 20.6 
202011.2 33.9 46.9 55.6 66.5 
202130.7 86.5 129.8 142.7 
202269.4 191.9 229.4 
202319.9 52.9 
202454.8 
Total672.4 
All outstanding liabilities before 2015, net of reinsurance
1.5 
Loss and LAE reserves, net of reinsurance
$536.7 
Marine & Energy
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2024
Accident Year2015201620172018201920202021202220232024Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2015$21.7 $17.4 $16.1 $13.3 $12.7 $12.8 $12.7 $12.9 $12.7 $12.7 $— 3,243 
201623.4 19.5 15.6 14.6 14.3 14.8 14.1 13.6 13.3 — 3,772 
201726.0 19.3 17.6 16.9 16.6 15.8 16.0 16.1 .4 4,139 
201825.4 19.9 17.4 17.8 17.3 17.7 16.7 .2 3,238 
201923.7 21.6 21.6 21.4 21.9 21.2 .5 2,393 
202029.7 27.1 28.5 27.4 27.1 .8 1,582 
202186.2 69.3 67.3 74.5 4.0 1,505 
2022149.7 153.6 156.0 31.9 1,968 
2023197.0 188.2 115.9 2,138 
2024239.9 182.6 1,485 
Total$765.7 
Marine & Energy
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2015201620172018201920202021202220232024
Unaudited
2015$4.0 $7.8 $9.6 $11.0 $10.4 $10.5 $10.9 $11.5 $11.6 $11.6 
20165.5 10.0 12.6 13.0 13.1 13.7 13.4 13.4 13.4 
20175.1 11.1 12.8 14.0 14.1 14.1 14.0 14.3 
20182.6 12.4 13.9 14.6 15.3 15.3 15.4 
20193.3 10.6 12.6 14.3 15.3 18.1 
20203.1 12.7 16.0 18.5 21.9 
20216.3 24.3 38.2 51.9 
202212.2 66.2 97.7 
202310.5 42.1 
202420.9 
Total307.3 
All outstanding liabilities before 2015, net of reinsurance4.7 
Loss and LAE reserves, net of reinsurance
$463.1 
Specialty
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2024
Accident Year2015201620172018201920202021202220232024Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2015$17.3 $14.6 $12.3 $10.7 $11.0 $11.2 $11.1 $8.9 $8.1 $11.3 $.3 1,841
201618.2 14.3 10.9 11.3 11.8 11.8 8.9 8.5 12.3 .4 1,936
201717.9 12.8 11.9 11.4 11.6 10.6 10.3 10.9 .4 2,195
201814.4 16.2 16.6 15.9 14.8 15.7 16.6 .4 2,122
201921.6 19.4 18.6 25.6 30.1 19.9 .3 2,387
202023.7 22.8 18.6 19.5 16.7 .9 2,017
202170.3 62.1 51.4 43.8 10.0 1,725
2022180.3 174.8 168.4 77.0 1,496
2023214.5 197.4 94.4 1,641
2024218.7 141.6 1,292
Total$716.0 
Specialty
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2015201620172018201920202021202220232024
Unaudited
2015$4.0 $7.0 $7.6 $8.0 $8.0 $8.1 $8.1 $6.4 $6.2 $9.3 
20163.2 7.9 9.1 9.9 10.3 10.3 8.5 8.3 11.7 
20173.1 6.5 8.3 8.5 8.5 9.2 8.9 9.6 
20182.7 8.2 9.9 10.4 11.8 13.0 14.1 
20194.8 6.9 7.4 18.2 25.1 17.6 
20205.0 10.5 12.9 17.7 17.7 
20215.0 23.9 35.5 34.6 
202216.0 61.7 82.5 
202318.4 75.2 
202427.3 
Total299.6 
All outstanding liabilities before 2015, net of reinsurance
.6 
Loss and LAE reserves, net of reinsurance
$417.0 
Casualty-Active
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2024
Accident Year2015201620172018201920202021202220232024Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2015$9.6 $9.7 $8.2 $8.1 $7.4 $7.1 $7.0 $7.3 $7.5 $7.4 $.5 1,306 
20168.8 8.3 8.9 9.0 9.1 9.2 9.2 10.1 11.8 .5 1,588 
201711.5 11.7 10.8 9.3 9.0 10.5 10.6 10.9 .8 1,667 
201812.9 13.3 11.1 10.8 8.6 9.1 9.4 1.1 1,147 
201914.8 13.7 12.3 10.6 11.4 13.0 1.7 1,019 
202013.5 12.0 10.8 9.2 8.8 2.0 665 
202121.4 22.4 16.6 16.3 6.4 961 
202232.9 38.0 34.6 27.9 1,558 
202360.9 65.9 57.4 1,792 
202459.5 54.9 1,170 
Total$237.6 
Casualty-Active
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2015201620172018201920202021202220232024
Unaudited
2015$1.8 $2.4 $3.2 $4.4 $4.7 $4.9 $5.1 $5.5 $6.1 $6.3 
2016.2 1.0 2.3 4.0 4.6 5.3 6.5 8.1 9.7 
2017.8 1.7 2.7 3.4 4.2 5.7 7.5 8.3 
2018.3 1.4 3.5 4.3 4.3 6.2 7.1 
2019.3 1.4 2.3 3.0 5.7 8.3 
2020.5 1.0 2.0 3.3 5.3 
2021.5 .9 3.1 9.6 
2022.4 1.5 2.4 
2023.9 5.6 
20241.9 
Total64.5 
All outstanding liabilities before 2015, net of reinsurance
4.9 
Loss and LAE reserves, net of reinsurance
$178.0 
Casualty-Runoff
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2024
Accident Year2015201620172018201920202021202220232024Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2015$36.4 $31.9 $33.1 $36.6 $36.3 $37.3 $36.7 $39.1 $40.2 $39.4 $1.9 1,950 
201632.4 32.1 40.3 38.4 38.7 38.4 37.6 37.3 37.4 2.0 2,150 
201730.5 33.8 31.3 32.0 31.5 29.8 28.2 28.4 2.3 1,604 
201833.5 28.1 27.2 26.5 26.1 27.9 27.7 3.4 1,280 
201926.4 23.2 23.3 24.8 23.5 23.6 5.2 973 
202015.8 12.2 13.8 10.9 9.5 2.9 567 
202110.4 7.0 5.4 4.5 1.8 283 
2022.8 2.6 2.5 1.6 80 
20232.7 3.6 2.4 40 
20241.3 .7 22 
Total$177.9 
Casualty-Runoff
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2015201620172018201920202021202220232024
Unaudited
2015$4.3 $8.2 $14.5 $21.4 $24.6 $27.3 $28.9 $33.0 $35.3 $35.8 
20163.9 10.1 17.7 22.7 25.3 27.8 28.7 30.9 32.4 
20173.2 9.4 14.6 18.4 21.3 22.5 22.8 23.5 
20183.4 7.4 12.6 14.9 16.2 18.2 21.3 
20193.3 5.8 7.8 12.1 15.1 15.8 
2020.8 1.3 3.1 6.0 6.3 
2021.5 1.7 1.8 2.3 
2022.3 .5 .7 
2023.9 1.0 
2024.5 
Total139.6 
All outstanding liabilities before 2015, net of reinsurance
28.2 
Loss and LAE reserves, net of reinsurance
$66.5 
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
23.8%32.8%18.3%6.4%2.8%1.3%0.5%0.8%0.1%0.1%
Marine & Energy
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
12.9%30.5%19.5%8.0%4.9%6.9%0.3%0.4%(0.3)%0.1%
Specialty
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
19.3%30.8%10.9%4.9%7.3%2.5%1.5%1.8%(1.3)%1.1%
Casualty-Active
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
4.4%8.4%11.6%14.4%9.5%11.7%6.3%4.8%3.7%3.1%
Casualty-Runoff
Average Annual Percentage Payout of Incurred Loss and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
8.8%14.1%15.6%14.8%8.5%6.8%5.9%4.1%3.1%2.0%