Gas Reserves - Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Gas Reserves [Abstract]
Gas reserves	$ 16,072	$ 15,926	$ 15,707
Gas reserves, non-current	171,464	171,610	171,834
Less: Accumulated amortization	79,444	71,426	63,548
Total gas reserves	108,092	116,110	123,993
Less: Deferred taxes on gas reserves	31,074	28,119	26,737
Net investment in gas reserves	$ 77,018	$ 87,991	$ 97,256