Oil and Gas Producing Activities (Unaudited) - Results of operations for oil and gas producing activities (Details) - USD ($) $ in Thousands	4 Months Ended	12 Months Ended
	Apr. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Revenues	$ 1,427	$ 3,410	$ 5,063
Costs and expenses:
Production	(951)	(3,337)	(7,022)
Producing property impairment	0	(30)	(10,324)
Exploration	0	0	(1,667)
Depreciation, depletion and amortization	(423)	(1,546)	(5,066)
Operating loss before income taxes	53	(1,503)	(19,016)
Income tax benefit (expense)	(19)	526	6,656
Operating earnings (losses)	$ 34	$ (977)	$ (12,360)