Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets:
Investments in marketable securities	$ 32,724	$ 36,993
Derivatives	998	3,955
Fund for asset retirement obligations	142,266	135,993
Total assets	175,988	176,941
Liabilities:
Derivatives	165
Level 1
Assets:
Investments in marketable securities	28,773	28,301
Derivatives		0
Fund for asset retirement obligations	142,266	135,993
Total assets	171,039	164,294
Liabilities:
Derivatives		0
Level 2
Assets:
Investments in marketable securities	3,951	8,692
Derivatives		2,655
Total assets	3,951	11,347
Liabilities:
Derivatives	165
Level 3
Assets:
Investments in marketable securities	0	0
Derivatives	998	1,300
Fund for asset retirement obligations	0	0
Total assets	998	1,300
Liabilities:
Derivatives	$ 0	$ 0