The changes in the workers' compensation liability, including current and long-term liability balances, for each of the periods presented were as follows:

(1)

Our estimate of the liability for the present value of current workers′ compensation benefits is based on our actuarial calculations. Our actuarial calculations are based on a blend of actuarial projection methods and numerous assumptions including claims development patterns, mortality, medical costs and interest rates. The valuation gain in 2021 is due to an increase in the discount rate from 1.95% on December 31, 2020 to 2.34% on June 30, 2021. We conducted a mid-year 2020 review of our actuarial assumptions, which resulted in a valuation loss in 2020 due to unfavorable changes in claims development and a decrease in the discount rate from 2.81% on December 31, 2019 to 2.05% on June 30, 2020.

We limit our exposure to traumatic injury claims by purchasing a high deductible insurance policy that starts paying benefits after deductibles for a claim have been met. The deductible level may vary by claim year. Our workers' compensation liability above is presented on a gross basis and does not include our expected receivables on our insurance policy. Our receivables for traumatic injury claims under this policy as of June 30, 2021 are $7.1 million and are included in Other long-term assets on our condensed consolidated balance sheet.

Certain of our mine operating entities are liable under state statutes and the Federal Coal Mine Health and Safety Act of 1969, as amended, to pay pneumoconiosis, or black lung, benefits to eligible employees and former employees and their dependents. Components of the net periodic benefit cost for each of the periods presented are as follows:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2021		2020		2021		2020

	(in thousands)
Service cost	$	989	$	883	$	2,020	$	1,761
Interest cost (1)		637		749		1,273		1,499
Net amortization (1)		1,043		(171)		2,086		(343)
Net periodic benefit cost	$	2,669	$	1,461	$	5,379	$	2,917

(1)	Interest cost and net amortization are included in the Other expense line item within our condensed consolidated statements of income.