Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Reserve Quantities [Line Items]
Gas reserves	$ 16,916	$ 15,704	$ 16,218
Gas reserves, non-current	170,391	171,832	171,318
Less: Accumulated amortization	99,835	87,779	83,442
Total gas reserves	87,472	99,757	104,094
Less: Deferred taxes on gas reserves	19,377	22,712	29,298
Net investment in gas reserves	68,095	77,045	74,796
Jonah Energy LLC
Reserve Quantities [Line Items]
Total Gas Reserves Investment	$ 5,000	$ 5,800	$ 6,000