Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets:
Investments in marketable securities	$ 30,447	$ 36,993
Derivatives	1,281
Fund for asset retirement obligations	140,413
Total assets	172,141
Level 1
Assets:
Investments in marketable securities	26,469
Derivatives	0
Fund for asset retirement obligations	140,413
Total assets	166,882
Liabilities:
Derivatives	0
Level 2
Assets:
Investments in marketable securities	3,978
Derivatives	46
Total assets	4,024
Level 3
Assets:
Investments in marketable securities	0
Derivatives	1,235
Fund for asset retirement obligations	0
Total assets	1,235
Liabilities:
Derivatives	$ 0