The Company's reserves for losses and loss expenses are comprised of case reserves and incurred but not reported liabilities ("IBNR"). When a claim is reported, a case reserve is established for the estimated ultimate payment based upon known information about the claim. As more information about the claim becomes available over time, case reserves are adjusted up or down as appropriate. Reserves are also established on an aggregate basis to provide for IBNR liabilities and expected loss reserve development on reported claims.

Loss reserves included in the Company’s financial statements represent management’s best estimates based upon an actuarially derived point estimate and other considerations. The Company uses a variety of actuarial techniques and methods to derive an actuarial point estimate for each operating unit. These methods include paid loss development, incurred loss development, paid and incurred Bornhuetter-Ferguson methods and frequency and severity methods. In circumstances where one actuarial method is considered more credible than the others, that method is used to set the point estimate. The actuarial point estimate may also be based on a judgmental weighting of estimates produced from each of the methods considered. Industry loss experience is used to supplement the Company’s own data in selecting “tail factors” in areas where the Company’s own data is limited. The actuarial data is analyzed by line of business, coverage and accident or policy year, as appropriate, for each operating unit.

The establishment of the actuarially derived loss reserve point estimate also includes consideration of qualitative factors that may affect the ultimate losses. These qualitative considerations include, among others, the impact of re-underwriting initiatives, changes in the mix of business, changes in distribution sources and changes in policy terms and conditions.

The key assumptions used to arrive at the best estimate of loss reserves are the expected loss ratios, rate of loss cost inflation, and reported and paid loss emergence patterns. Expected loss ratios represent management’s expectation of losses at the time the business is priced and written, before any actual claims experience has emerged. This expectation is a significant determinant of the estimate of loss reserves for recently written business where there is little paid or incurred loss data to consider. Expected loss ratios are generally derived from historical loss ratios adjusted for the impact of rate changes, loss cost trends and known changes in the type of risks underwritten. Expected loss ratios are estimated for each key line of business within each operating unit. Expected loss cost inflation is particularly important for the long-tail lines, such as excess casualty, and claims with a high medical component, such as workers’ compensation. Reported and paid loss emergence patterns are used to project current reported or paid loss amounts to their ultimate settlement value. Loss development factors are based on the historical emergence patterns of paid and incurred losses, and are derived from the Company’s own experience and industry data. The paid loss emergence pattern is also significant to excess and assumed workers’ compensation reserves because those reserves are discounted to their estimated present value based upon such estimated payout patterns.

Loss frequency and severity are measures of loss activity that are considered in determining the key assumptions described in our discussion of loss and loss expense reserves, including expected loss ratios, rate of loss cost inflation and reported and paid loss emergence patterns. Loss frequency is a measure of the number of claims per unit of insured exposure, and loss severity is a measure of the average size of claims. Factors affecting loss frequency include the effectiveness of loss controls and safety programs and changes in economic activity or weather patterns. Factors affecting loss severity include changes in policy limits, retentions, rate of inflation and judicial interpretations.

Another factor affecting estimates of loss frequency and severity is the loss reporting lag, which is the period of time between the occurrence of a loss and the date the loss is reported to the Company. The length of the loss reporting lag affects our ability to accurately predict loss frequency (loss frequencies are more predictable for lines with short reporting lags) as well as the amount of reserves needed for incurred but not reported losses (less IBNR is required for lines with short reporting lags). As a result, loss reserves for lines with short reporting lags are likely to have less variation from initial loss estimates. For lines with short reporting lags, which include auto, primary workers’ compensation, other liability (claims-made) and property business, the key assumption is the loss emergence pattern used to project ultimate loss estimates from known losses paid or reported to date. For lines of business with long reporting lags, which include other liability (occurrence), products liability, excess workers’ compensation and liability reinsurance, the key assumption is the expected loss ratio since there is often little paid or incurred loss data to consider. Historically, the Company has experienced less variation from its initial loss estimates for lines of business with short reporting lags than for lines of business with long reporting lags.


	September 30,
(In thousands)	2024		2023
Net reserves at beginning of period	$	15,661,820	$	14,248,879


Net provision for losses and loss expenses:
Claims occurring during the current year (1)	5,229,468		4,694,554
Increase in estimates for claims occurring in prior years (2) (3)	15,279		27,186
Loss reserve discount accretion	25,587		22,862
Total	5,270,334		4,744,602
Net payments for claims:
Current year	795,792		743,416
Prior years	3,164,720		2,915,979
Total	3,960,512		3,659,395
Foreign currency translation	19,659		(48,162)
Net reserves at end of period	16,991,301		15,285,924
Ceded reserves at end of period	3,164,009		2,987,386
Gross reserves at end of period	$	20,155,310	$	18,273,310

(3) For certain retrospectively rated insurance policies and reinsurance agreements, reserve development is offset by additional or return premiums. Favorable development, net of additional and return premiums, was $3 million and adverse development was $20 million for the nine months ended September 30, 2024 and 2023, respectively.

The ultimate net impact of COVID-19 on the Company’s reserves remains uncertain. As of September 30, 2024, the Company had recognized losses for COVID-19-related claims activity, net of reinsurance, of approximately $382 million, of which $325 million relates to the Insurance segment and $57 million relates to the Reinsurance & Monoline Excess segment. Such $382 million of COVID-19-related losses included $379 million of reported losses and $3 million of IBNR.

For the Insurance segment, the adverse development during the first nine months of 2024 was driven by commercial auto liability and other liability (mainly umbrella and excess liability), which was largely offset by favorable development for workers’ compensation and professional liability. The adverse commercial auto liability development was concentrated in accident years 2020 through 2023. The other liability development was mainly driven by umbrella and excess liability claims, and was focused in accident years 2017 through 2021. A significant portion of the umbrella and excess liability development related to underlying commercial auto exposures. The Company believes that commercial auto-related claims are being particularly impacted by social inflation, which is contributing to an increase in the frequency of large losses beyond expectations. Social inflation can include higher settlement demands from plaintiffs, use of aggressive actions by the plaintiffs’ bar such as litigation funding, negative public sentiment towards large businesses and corporations, and erosion of tort reforms, among other factors.

The favorable workers’ compensation development for the Insurance segment was mainly related to accident years 2017 through 2023, while the favorable professional liability development was mainly in accident years 2020 through 2022. For workers' compensation, favorable reported claim frequency, below expectations, continued to be the main driver of the favorable reserve development. For professional liability, reported loss experience for accident years 2020 through 2022 was better than expected, which drove the favorable reserve development. These accident years also feature business written at peak pricing levels, which the Company now believes will result in higher profitability than initially anticipated.

For the Reinsurance & Monoline Excess segment, the favorable development during the first nine months of 2024 was driven mainly by favorable development in excess workers’ compensation, partially offset by adverse development in the non-proportional reinsurance assumed liability line of business. The favorable excess workers’ compensation development was

driven by continued lower claim frequency and reported losses relative to expectations, and to favorable claim settlements spread across many prior accident years. The unfavorable development for non-proportional reinsurance was concentrated mainly in accident years 2015 through 2019 and was associated primarily with our U.S. and U.K. excess general liability reinsurance businesses, including coverage for cedants insuring construction projects.

During the nine months ended September 30, 2023, adverse prior year development (net of additional and return premiums) of $20 million included $21 million of adverse development for the Insurance segment, partially offset by $1 million of favorable development for the Reinsurance & Monoline Excess segment.

Such adverse development during the nine months ended September 30, 2023 was concentrated in the first quarter, with $24 million of adverse development (net of additional and return premiums) recorded in the first quarter, partially offset by favorable development totaling $4 million recorded during the second and third quarters. The recorded adverse development during the first quarter in both business segments was due to property catastrophe losses related to 2022 events which were still being adjusted and settled. In particular, losses related to U.S. winter storms which occurred during the month of December 2022 were a significant driver of the development, as information gathering and evaluation of many of these losses were still ongoing into the new year.

For the Insurance segment, in addition to the property prior year adverse development discussed above, the adverse development during the nine months ended September 30, 2023 included adverse prior year development on casualty lines for the 2016 through 2019 accident years, which was largely offset by favorable prior year development on casualty lines for the 2020 through 2022 accident years. The adverse development on the 2016 through 2019 accident years was concentrated in the general liability line of business, and to a lesser degree professional liability, including medical professional, and commercial auto liability. The development, which particularly impacted business attaching excess of primary policy limits, was driven by a larger than expected number of large losses reported. The Company believes social inflation is contributing to an increase in the frequency of large losses for these accident years. Social inflation can include higher settlement demands from plaintiffs, use of tactics such as litigation funding by the plaintiffs’ bar, negative public sentiment towards large businesses and corporations, and erosion of tort reforms, among others.

The favorable prior year development on casualty lines for the 2020 through 2022 accident years in the Insurance segment was concentrated in the professional liability, workers’ compensation, and general liability lines of business. Due to uncertainty regarding incurred loss frequency and severity in light of ongoing social inflation and the impacts of the COVID-19 pandemic, the Company set its initial loss ratios for the 2020 through 2022 accident years prudently, and largely maintained these estimates through the end of each respective accident year. The reported loss experience to date for these lines of business for the 2020 through 2022 accident years has been significantly better than was expected, and the Company began to react to this favorable emergence as the accident years matured beyond the age of 12 months. However, commercial auto liability experienced adverse prior year development for the 2020 through 2022 accident years, partially offsetting the favorable development discussed above, which was driven by a larger than expected number of large losses reported.

For the Reinsurance & Monoline Excess segment, the favorable development during the nine months ended September 30, 2023 was driven mainly by favorable development in excess workers’ compensation, largely offset by adverse development in property (discussed above), non-proportional reinsurance assumed liability, and commercial auto liability lines of business. The favorable excess workers’ compensation development was driven by continued lower claim frequency and reported losses relative to our expectations, and to favorable claim settlements. The favorable development was spread across many prior accident years. The adverse development on reinsurance assumed liability was associated primarily with our U.S. assumed reinsurance business, and related to accounts reinsuring excess and umbrella business and construction projects. The adverse development was concentrated mainly in accident years 2017 through 2020. The adverse development on commercial auto liability was concentrated in the 2022 accident year and related to commercial auto program business.