Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Reserve Quantities [Line Items]
Gas reserves	$ 13,646	$ 15,278	$ 17,206
Gas reserves, non-current	173,661	172,029	170,101
Less: Accumulated amortization	132,202	123,635	113,930
Total gas reserves	55,105	63,672	73,377
Less: Deferred taxes on gas reserves	13,324	15,515	26,995
Net investment in gas reserves	41,781	48,157	46,382
Jonah Energy LLC
Reserve Quantities [Line Items]
Total Gas Reserves Investment	$ 3,400	$ 3,800	$ 4,300