Fair Value Measurements - Financial assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Assets:
Investments in marketable securities	$ 34,252	$ 32,724
Fund for asset retirement obligations	144,146
Total assets	178,398
Level 1
Assets:
Investments in marketable securities	31,296
Fund for asset retirement obligations	144,146
Total assets	175,442
Level 2
Assets:
Investments in marketable securities	2,956
Fund for asset retirement obligations	0
Total assets	2,956
Level 3
Assets:
Investments in marketable securities	0
Fund for asset retirement obligations	0
Total assets	$ 0