Oil and Gas Producing Activities (Unaudited) - Proved developed and undeveloped oil and gas reserves (Details) bbl in Thousands, MMcf in Thousands	4 Months Ended	12 Months Ended
	Apr. 30, 2017 bbl MMcf	Dec. 31, 2016 bbl MMcf	Dec. 31, 2015 bbl MMcf
Natural Gas [Member]
Total proved reserves:
Beginning of year | MMcf	4,214	4,835	5,611
Revisions of previous estimates | MMcf	0	65	27
Crude oil and natural gas reserves sold | MMcf	(4,067)	(175)	0
Extensions, discoveries and other reserve additions | MMcf	42	151	86
Production | MMcf	(189)	(662)	(889)
End of year | MMcf	0	4,214	4,835
Crude Oil [Member]
Total proved reserves:
Beginning of year | bbl	187	226	318
Revisions of previous estimates | bbl	0	24	(2)
Crude oil and natural gas reserves sold | bbl	(170)	(4)	(3)
Extensions, discoveries and other reserve additions | bbl	6	18	13
Production | bbl	(23)	(77)	(100)
End of year | bbl	0	187	226