Supplementary Information on Oil and Gas Operations—Unaudited - Proved Reserves (Details) Mcfe in Thousands, MMcf in Thousands, MBbls in Thousands	12 Months Ended
	Dec. 31, 2018 Mcfe MBbls MMcf	Dec. 31, 2017 Mcfe MBbls MMcf	Dec. 31, 2016 Mcfe MBbls MMcf
Oil
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance | MBbls	1,848	1,397	1,806
Revisions of previous estimates | MBbls	130	308	247
Extensions, discoveries and other additions | MBbls	41	777	0
Purchase of producing properties | MBbls		48
Sale of reserves in place | MBbls	(507)	(90)	(154)
Production | MBbls	(326)	(592)	(502)
Proved reserves ending balance | MBbls	1,186	1,848	1,397
Proved developed reserves | MBbls	567	1,078	1,212
Proved undeveloped reserve | MBbls	619	770	185
Mcfe of natural gas liquids
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	19,421	26,575	34,826
Revisions of previous estimates	(1,469)	(7,269)	(4,380)
Extensions, discoveries and other additions	2,929	4,565	0
Purchase of producing properties		0
Sale of reserves in place	(486)	0	0
Production	(3,373)	(4,450)	(3,871)
Proved reserves, ending balance	17,022	19,421	26,575
Proved undeveloped reserve	10,220	12,564	13,073
Proved developed reserves	6,802	6,857	13,502
Natural Gas
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance | MMcf	125,438	80,524	132,344
Revisions of previous estimates | MMcf	2,737	381	(11,854)
Extensions, discoveries and other additions | MMcf	7,947	64,704	1,485
Purchase of producing properties | MMcf		473
Sale of reserves in place | MMcf	(13,945)	(1,033)	(24,834)
Production | MMcf	(16,013)	(19,611)	(16,617)
Proved reserves ending balance | MMcf	106,164	125,438	80,524
Proved developed reserves | MMcf	47,516	57,409	47,349
Proved undeveloped reserve | MMcf	58,648	68,029	33,175
Oil and Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	155,947	115,481	178,004
Revisions of previous estimates	2,044	(5,040)	(14,748)
Extensions, discoveries and other additions	11,121	73,931	1,485
Purchase of producing properties		761
Sale of reserves in place	(17,472)	(1,573)	(25,759)
Production	(21,335)	(27,613)	(23,501)
Proved reserves, ending balance	130,305	155,947	115,481
Proved undeveloped reserve	61,143	76,441	67,694
Proved developed reserves	69,162	79,506	47,787