Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Reserve Quantities [Line Items]
Gas reserves	$ 16,157	$ 16,647	$ 15,124
Gas reserves, non-current	171,150	170,660	172,412
Less: Accumulated amortization	109,243	104,463	91,852
Total gas reserves	78,064	82,844	95,684
Less: Deferred taxes on gas reserves	19,638	20,071	22,115
Net investment in gas reserves	58,426	62,773	73,569
Jonah Energy LLC
Reserve Quantities [Line Items]
Total Gas Reserves Investment	$ 4,500	$ 4,800	$ 5,600