Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Assets:
Investments in marketable securities	$ 35,583	$ 32,724
Fund for asset retirement obligations	146,010	142,266
Total assets	181,593	174,990
Level 1
Assets:
Investments in marketable securities	17,328	28,773
Fund for asset retirement obligations	146,010	142,266
Total assets	163,338	171,039
Level 2
Assets:
Investments in marketable securities	18,255	3,951
Fund for asset retirement obligations	0
Total assets	18,255	$ 3,951
Level 3
Assets:
Investments in marketable securities	0
Fund for asset retirement obligations	0
Total assets	$ 0