The reconciliation of the net incurred and paid claims development tables to the liability for claims and claim adjustment expenses in the consolidated balance sheet at December 31, 2020 and 2019 is as follows. 2020 2019 Net outstanding liabilities Commercial Liability $ 420,698 $ 355,148 Workers' Compensation 168,859 154,409  Occupational Accident 15,039 19,840 Physical Damage 14,561 11,676  Professional Liability Assumed 11,958 16,079 Other short-duration insurance lines 10,016 10,478  Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 641,131 567,630 Reinsurance recoverable on unpaid claims Commercial Liability 209,126 209,152  Workers' Compensation 211,421 182,908 Occupational Accident 500 1,979  Physical Damage 1,054 655 Other short-duration insurance lines 2,267 3,611  Reinsurance recoverable on unpaid losses at the end of the year 424,368 398,305 Unallocated claims adjustment expenses 24,170 22,370 Total gross liability for unpaid claims and claims adjustment expense $ 1,089,669 $ 988,305 Years 1 2 3 4 5 6 7 8 9 10 Commercial Liability 8.8% 25.2% 22.8% 19.8% 8.9% 5.4% 3.3% 0.8% 1.1% – Workers' Compensation 16.9% 26.5% 16.1% 10.1% 6.2% 3.8% 3.1% 2.0% 1.1% 0.6% Occupational Accident 36.2% 40.7% 11.2% 4.1% 1.0% 1.6% (0.1)% 0.4% 0.6% 0.3% Physical Damage 81.6% 16.3% (0.4)% N/A N/A N/A N/A N/A N/A N/A Professional Liability Assumed 1.4% 3.2% 11.8% 27.6% 16.9% 14.9% 9.0% 4.1% 2.6% 1.7% Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (Supplementary Information and Unaudited) The following is supplementary information about average historical claims duration as of December 31, 2020: Reserve methodologies for incurred but not reported losses The Company uses both standard actuarial techniques common to most insurance companies as well as proprietary techniques developed by the Company in connection with its specialty business products. For its short-tail lines of business, the Company uses predominantly the incurred or paid loss development factor methods. The Company has found that the use of accident quarter loss development triangles, rather than those based upon accident year, are most responsive to claim settlement trends and fluctuations in premium exposure for its short-tail lines. A minimum of 12 running accident quarters is used to project the reserve necessary for incurred but not reported losses for its short-tail lines. The Company also uses the loss development factor approach for its long-tail lines of business, including workers' compensation. A minimum of 15 accident years is included in the loss development triangles used to calculate link ratios and the selected loss development factors used to determine the reserves for incurred but not reported losses. Significant emphasis is placed on the use of tail factors for the Company's long-tail lines of business. 68