Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Assets:
Investments in marketable securities	$ 32,116	$ 36,993
Derivatives	4,628
Fund for asset retirement obligations	137,134
Total assets	173,878
Level 1
Assets:
Investments in marketable securities	25,388
Derivatives	0
Fund for asset retirement obligations	137,134
Total assets	162,522
Liabilities:
Derivatives	0
Level 2
Assets:
Investments in marketable securities	6,728
Derivatives	3,527
Total assets	10,255
Level 3
Assets:
Investments in marketable securities	0
Derivatives	1,101
Fund for asset retirement obligations	0
Total assets	1,101
Liabilities:
Derivatives	$ 0