Oil and Gas Producing Activities (Unaudited) - Cost incurred in oil and gas exploration and development activities (Details) - USD ($) $ in Thousands	4 Months Ended	12 Months Ended
	Apr. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Property acquisition costs:
Unproved	$ 4	$ 32	$ 348
Proved	0	0	0
Exploration costs:
Expensed	5	291	1,667
Capitalized	0	0	0
Development costs	1,815	0	370
Total costs incurred	$ 1,824	$ 323	$ 2,385