Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Assets:
Investments in marketable securities	$ 36,347	$ 32,724
Fund for asset retirement obligations	147,932	142,266
Total assets	184,279	174,990
Level 1
Assets:
Investments in marketable securities	14,912	28,773
Fund for asset retirement obligations	147,932	142,266
Total assets	162,844	171,039
Level 2
Assets:
Investments in marketable securities	21,435	3,951
Fund for asset retirement obligations	0
Total assets	21,435	$ 3,951
Level 3
Assets:
Investments in marketable securities	0
Fund for asset retirement obligations	0
Total assets	$ 0