Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Assets:
Investments in marketable securities	$ 33,607	$ 36,993
Derivatives	230
Fund for asset retirement obligations	138,657
Total assets	172,494
Level 1
Assets:
Investments in marketable securities	28,645
Derivatives	0
Fund for asset retirement obligations	138,657
Total assets	167,302
Liabilities:
Derivatives	0
Level 2
Assets:
Investments in marketable securities	4,962
Total assets	4,962
Level 3
Assets:
Investments in marketable securities	0
Derivatives	230
Fund for asset retirement obligations	0
Total assets	230
Liabilities:
Derivatives	$ 0