Oil and Gas Producing Activities (Unaudited) - Proved developed and undeveloped oil and gas reserves (Details) bbl in Thousands, MMcf in Thousands	12 Months Ended
	Dec. 31, 2017 MMcf bbl	Dec. 31, 2016 bbl MMcf
Natural Gas
Total proved reserves:
Beginning of year | MMcf	4,214	4,835
Revisions of previous estimates | MMcf	0	65
Crude oil and natural gas reserves sold | MMcf	(4,067)	(175)
Extensions, discoveries and other reserve additions | MMcf	42	151
Production | MMcf	(189)	(662)
End of year | MMcf	0	4,214
Crude Oil
Total proved reserves:
Beginning of year | bbl	187	226
Revisions of previous estimates | bbl	0	24
Crude oil and natural gas reserves sold | bbl	(170)	(4)
Extensions, discoveries and other reserve additions | bbl	6	18
Production | bbl	(23)	(77)
End of year | bbl	0	187