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Loss and Loss Adjustment Expense Reserves (Tables)
 12 Months Ended
Dec. 31, 2023
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, 2023, 2022 and 2021:
Year Ended December 31,
Millions202320222021
Gross beginning balance$1,296.5 $894.7 $696.0 
Less: beginning reinsurance recoverable on unpaid losses (1)
(505.0)(428.9)(433.4)
Net loss and LAE reserves791.5 465.8 262.6 
Losses and LAE incurred relating to:
      Current year losses706.9 588.1 336.3 
      Prior year losses19.9 (51.7)(21.5)
Net incurred losses and LAE726.8 536.4 314.8 
Loss and LAE paid relating to:
Current year losses(51.3)(98.9)(43.9)
   Prior year losses(356.6)(158.6)(61.6)
Net paid losses and LAE(407.9)(257.5)(105.5)
Change in TPC Providers’ participation (2)
145.4 57.5 (2.2)
Foreign currency translation and other adjustments to loss and LAE reserves8.5 (10.7)(3.9)
Net ending balance1,264.3 791.5 465.8 
Plus: ending reinsurance recoverable on unpaid losses (3)
340.8 505.0 428.9 
Gross ending balance$1,605.1 $1,296.5 $894.7 
(1) The beginning reinsurance recoverable on unpaid losses includes amounts attributable to TPC Providers of $145.4, $276.8 and $319.2 as of December 31, 2022, 2021 and 2020.
(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 $0.0, $145.4 and $276.8 as of December 31, 2023, 2022 and 2021.
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, 2023 and 2022:

Year Ended December 31,


Millions		20232022
Property and Accident & Health$358.7 $258.2 
Specialty339.7 204.3 
Marine & Energy331.7 196.4 
Casualty-Active 137.1 71.5 
Casualty-Runoff81.3 60.8 
WM Outrigger and Other15.8 .3 
   Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses1,264.3 791.5 
Plus: Reinsurance recoverables on unpaid losses (1)
Property and Accident & Health (2)
126.9 224.6 
Specialty77.6 97.2 
Marine & Energy59.1 79.8 
Casualty-Active74.8 49.9 
Casualty-Runoff2.4 53.5 
Total Reinsurance recoverables on unpaid losses (1) (2)
340.8 505.0 
Total unpaid loss and LAE reserves$1,605.1 $1,296.5 
(1) The reinsurance recoverables on unpaid losses include amounts attributable to TPC Providers of $0.0 and $145.4 as of December 31, 2023 and 2022.
(2) The reinsurance recoverables on unpaid losses exclude $15.6 ceded by Ark to WM Outrigger Re as of December 31, 2023, which eliminate in White Mountains’s consolidated financial statements.
Schedule of third party participation of LAE reserves by year of account
The following table summarizes the participation of Ark’s TPC Providers by year of account:

2014201520162017201820192020202120222023
TPC Providers’
   Participation		66.2 %70.0 %59.6 %60.0 %57.6 %58.3 %42.8 %— %— %— %
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, 2023
Accident Year2014201520162017201820192020202120222023
Total IBNR plus expected development on reported claims
Cumulative number of reported claims
Unaudited
2014$32.8 $29.5 $29.4 $28.7 $28.5 $28.6 $28.6 $28.6 $28.6 $28.5 $.1 2,924
201519.3 18.3 17.3 16.3 16.1 16.2 15.9 15.8 16.1 .2 2,829
201622.5 17.7 18.5 18.6 18.6 18.8 18.8 18.7 .2 3,428
201731.3 38.1 45.6 44.6 43.2 42.7 44.2 15.3 4,613
201841.0 47.4 49.3 47.0 47.1 46.7 1.9 4,269
201934.2 31.4 27.2 24.0 23.4 .8 4,011
202077.6 75.8 75.1 78.4 12.4 4,617
2021171.8 155.5 167.3 5.7 3,465
2022244.3 270.5 14.3 3,899
2023214.2 156.9 2,963
Total$908.0 
Property and Accident & Health
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2014201520162017201820192020202120222023
Unaudited
2014$13.7 $25.1 $27.3 $27.6 $27.7 $27.9 $28.0 $28.0 $28.0 $28.0 
20156.9 12.3 13.5 14.7 14.7 14.9 15.1 15.1 15.5 
20168.6 13.2 16.5 16.9 17.0 17.3 17.9 18.1 
201716.9 26.0 31.8 33.0 29.8 27.5 25.7 
201815.7 32.4 40.3 40.3 41.1 43.1 
20196.8 16.8 18.5 18.6 19.4 
202011.3 34.3 47.3 56.1 
202130.9 87.1 131.3 
202270.9 193.9 
202320.1 
Total551.2 
All outstanding liabilities before 2014, net of reinsurance
1.9 
Loss and LAE reserves, net of reinsurance
$358.7 
Specialty
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2023
Accident Year2014201520162017201820192020202120222023Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2014$46.0 $44.2 $41.2 $40.8 $41.2 $43.8 $43.8 $43.7 $43.5 $43.5 $— 1,359
201517.4 14.8 12.4 10.8 11.1 11.3 11.3 9.0 8.2 (.1)1,841
201618.4 14.4 11.0 11.3 11.9 11.9 9.0 8.6 .1 1,932
201718.1 13.0 12.1 11.6 11.7 10.7 10.4 .1 2,191
201814.6 16.4 16.8 16.1 14.9 15.8 .4 2,117
201921.8 19.6 18.7 25.7 30.1 .2 2,368
202024.2 23.2 19.0 19.9 1.7 1,994
202171.2 62.6 51.7 16.4 1,696
2022181.1 176.6 97.2 1,407
2023215.6 151.6 1,258
Total$580.4 
Specialty
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2014201520162017201820192020202120222023
Unaudited
2014$26.5 $39.3 $40.1 $40.5 $41.1 $42.4 $43.2 $43.1 $43.4 $43.7 
20154.0 7.1 7.7 8.1 8.1 8.2 8.2 6.5 6.3 
20163.2 8.0 9.1 9.9 10.4 10.4 8.6 8.4 
20173.2 6.7 8.5 8.6 8.6 9.3 9.1 
20182.8 8.3 10.1 10.5 11.9 13.1 
20194.9 7.0 7.5 18.3 25.2 
20205.3 10.7 13.2 18.3 
20215.1 24.1 35.7 
202216.0 62.1 
202318.8 
Total240.7 
All outstanding liabilities before 2014, net of reinsurance
— 
Loss and LAE reserves, net of reinsurance
$339.7 
Specialty
Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
20.5%30.6%9.7%6.0%7.7%6.0%1.2%1.7%(3.1)%(0.5)%
Marine & Energy
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2023
Accident Year2014201520162017201820192020202120222023Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2014$34.4 $20.2 $17.2 $16.3 $14.1 $13.7 $14.0 $13.8 $13.9 $13.6 $(.3)2,572 
201521.8 17.5 16.3 13.4 12.8 13.0 12.8 13.1 12.9 .1 3,242 
201623.6 19.7 15.8 14.7 14.4 15.0 14.2 13.8 .1 3,770 
201726.2 19.5 17.7 17.1 16.8 15.9 16.1 .3 4,133 
201825.8 20.3 17.8 18.1 17.7 18.1 .5 3,225 
201923.9 21.8 21.8 21.6 22.1 .9 2,366 
202030.0 27.3 28.7 27.6 1.6 1,561 
202186.8 69.8 67.7 9.3 1,461 
2022150.2 154.2 50.4 1,802 
2023197.8 154.8 1,539 
Total$543.9 
Marine & Energy
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2014201520162017201820192020202120222023
Unaudited
2014$5.9 $12.3 $13.4 $14.2 $14.3 $13.6 $13.8 $13.6 $13.9 $13.8 
20154.0 7.9 9.7 11.1 10.5 10.6 11.0 11.6 11.7 
20165.6 10.1 12.7 13.1 13.3 13.9 13.6 13.6 
20175.1 11.2 12.9 14.2 14.2 14.2 14.1 
20182.7 12.7 14.3 15.0 15.6 15.6 
20193.4 10.7 12.7 14.5 15.5 
20203.2 12.8 16.2 18.7 
20216.3 24.5 38.5 
202212.3 66.4 
202310.6 
Total218.5 
All outstanding liabilities before 2013, net of reinsurance6.3 
Loss and LAE reserves, net of reinsurance
$331.7 
Casualty-Active
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2023
Accident Year2014201520162017201820192020202120222023Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2014$13.3 $9.4 $8.4 $8.2 $8.1 $7.7 $7.8 $7.6 $7.8 $7.7 $.3 1,393 
20159.6 9.8 8.2 8.1 7.4 7.2 7.0 7.3 7.5 .4 1,295 
20168.8 8.3 8.9 9.0 9.1 9.2 9.2 10.1 .5 1,553 
201711.6 11.7 10.8 9.4 9.1 10.5 10.7 1.1 1,632 
201812.9 13.3 11.1 10.9 8.6 9.2 1.4 1,098 
201914.8 13.7 12.4 10.6 11.4 2.6 954 
202013.5 12.1 10.9 9.2 4.3 605 
202121.4 22.4 16.6 12.4 842 
202233.0 38.1 34.4 1,237 
202361.0 57.9 1,065 
Total$181.5 
Casualty-Active
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2014201520162017201820192020202120222023
Unaudited
2014$1.3 $3.5 $4.2 $4.7 $5.2 $5.5 $5.9 $6.0 $6.2 $6.7 
20151.8 2.4 3.2 4.4 4.7 4.9 5.1 5.5 6.1 
2016.2 1.0 2.3 4.0 4.6 5.3 6.5 8.1 
2017.8 1.7 2.8 3.4 4.2 5.7 7.5 
2018.3 1.4 3.5 4.3 4.3 6.2 
2019.3 1.4 2.3 3.0 5.7 
2020.5 1.0 2.0 3.3 
2021.5 .9 3.1 
2022.4 1.6 
2023.9 
Total49.2 
All outstanding liabilities before 2014, net of reinsurance
4.8 
Loss and LAE reserves, net of reinsurance
$137.1 
Casualty-Runoff
$ in Millions
Incurred Loss and LAE, Net of Reinsurance
For the Years Ended December 31,As of December 31, 2023
Accident Year2014201520162017201820192020202120222023Total IBNR plus expected development on reported claimsCumulative number of reported claims
Unaudited
2014$49.2 $48.4 $51.0 $54.0 $57.6 $59.1 $59.1 $58.9 $58.7 $59.7 $1.9 1,947 
201536.7 32.1 33.3 36.8 36.5 37.5 36.8 39.3 40.3 2.0 2,000 
201632.6 32.2 40.4 38.6 38.8 38.6 37.7 37.5 2.3 2,158 
201730.6 34.0 31.4 32.1 31.6 29.9 28.3 2.9 1,603 
201833.7 28.2 27.3 26.6 26.2 28.0 4.1 1,277 
201926.6 23.3 23.4 24.9 23.6 5.8 971 
202015.9 12.3 13.9 10.9 3.9 566 
202110.5 7.0 5.5 3.0 282 
2022.8 2.6 1.8 77 
20232.7 1.6 40 
Total$239.1 
Casualty-Runoff
Millions
Cumulative Paid Loss and LAE, Net of Reinsurance
For the Years Ended December 31,
Accident Year2014201520162017201820192020202120222023
Unaudited
2014$6.5 $23.2 $29.6 $36.5 $43.2 $47.0 $48.7 $49.4 $51.9 $54.9 
20154.3 8.2 14.5 21.4 24.7 27.4 29.0 33.1 35.3 
20163.9 10.2 17.7 22.7 25.4 27.8 28.7 31.0 
20173.2 9.4 14.7 18.5 21.4 22.5 22.8 
20183.4 7.4 12.6 14.9 16.3 18.2 
20193.3 5.8 7.8 12.2 15.2 
2020.8 1.3 3.1 6.0 
2021.5 1.7 1.9 
2022.3 .5 
2023.9 
Total186.7 
All outstanding liabilities before 2014, net of reinsurance
28.9 
Loss and LAE reserves, net of reinsurance
$81.3 
Short-duration insurance contracts, schedule of historical claims duration
Property and Accident & Health
Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
25.8%35.5%20.1%6.1%1.3%1.1%0.4%0.3%0.1%—%
Marine & Energy
Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
14.2%34.9%19.4%6.1%4.3%6.5%0.3%0.3%(0.3)%0.1%
Casualty-Active
Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
4.8%8.8%14.2%11.3%8.8%10.9%6.1%4.6%2.0%3.2%
Casualty-Runoff
Average Annual Percentage Payout of Incurred Losses and LAE by Age, Net of Reinsurance
Unaudited
Years12345678910
8.6%14.0%15.5%14.8%8.6%7.0%5.5%4.2%3.0%2.1%