Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Gas Reserves [Abstract]
Gas reserves	$ 16,218	$ 15,926	$ 16,257
Gas reserves, non-current	171,318	171,610	171,280
Less: Accumulated amortization	83,442	71,426	67,304
Total gas reserves	104,094	116,110	120,233
Less: Deferred taxes on gas reserves	29,298	28,119	25,799
Net investment in gas reserves	$ 74,796	$ 87,991	$ 94,434