Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Gas Reserves [Abstract]
Gas reserves	$ 15,124	$ 15,704	$ 15,378
Gas reserves, non-current	172,412	171,832	172,158
Less: Accumulated amortization	91,852	87,779	75,528
Total gas reserves	95,684	99,757	112,008
Less: Deferred taxes on gas reserves	22,115	22,712	32,179
Net investment in gas reserves	$ 73,569	$ 77,045	$ 79,829