Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Reserve Quantities [Line Items]
Gas reserves	$ 16,392	$ 16,647	$ 16,916
Gas reserves, non-current	170,915	170,660	170,391
Less: Accumulated amortization	118,937	104,463	99,835
Total gas reserves	68,370	82,844	87,472
Less: Deferred taxes on gas reserves	13,293	20,071	19,377
Net investment in gas reserves	55,077	62,773	68,095
Jonah Energy LLC
Reserve Quantities [Line Items]
Total Gas Reserves Investment	$ 4,000	$ 4,800	$ 5,000