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, 2017 Mcfe MBbls MMcf	Dec. 31, 2016 Mcfe MBbls MMcf	Dec. 31, 2015 Mcfe MBbls MMcf
Oil
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance | MBbls	1,397	1,806	2,437
Revisions of previous estimates | MBbls	308	247	(211)
Extensions, discoveries and other additions | MBbls	777	0	163
Purchase of producing properties | MBbls	48
Sale of reserves in place | MBbls	(90)	(154)	(54)
Production | MBbls	(592)	(502)	(529)
Proved reserves ending balance | MBbls	1,848	1,397	1,806
Proved developed reserves | MBbls	1,078	1,212	1,549
Proved undeveloped reserve | MBbls	770	185	257
Mcfe of natural gas liquids
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	26,575	34,826	73,498
Revisions of previous estimates	(7,269)	(4,380)	(3,571)
Extensions, discoveries and other additions	4,565	0	16,078
Purchase of producing properties	0
Sale of reserves in place	0	0	(45,692)
Production	(4,450)	(3,871)	(5,487)
Proved reserves, ending balance	19,421	26,575	34,826
Proved undeveloped reserve	12,564	13,073	15,792
Proved developed reserves	6,857	13,502	19,034
Natural Gas
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance | MMcf	80,524	132,344	309,025
Revisions of previous estimates | MMcf	381	(11,854)	(9,852)
Extensions, discoveries and other additions | MMcf	64,704	1,485	45,645
Purchase of producing properties | MMcf	473
Sale of reserves in place | MMcf	(1,033)	(24,834)	(186,972)
Production | MMcf	(19,611)	(16,617)	(25,502)
Proved reserves ending balance | MMcf	125,438	80,524	132,344
Proved developed reserves | MMcf	57,409	47,349	78,533
Proved undeveloped reserve | MMcf	68,029	33,175	53,811
Oil and Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	115,481	178,004	397,148
Revisions of previous estimates	(5,040)	(14,748)	(14,698)
Extensions, discoveries and other additions	73,931	1,485	62,702
Purchase of producing properties	761
Sale of reserves in place	(1,573)	(25,759)	(232,988)
Production	(27,613)	(23,501)	(34,160)
Proved reserves, ending balance	155,947	115,481	178,004
Proved undeveloped reserve	76,441	67,694	103,615
Proved developed reserves	79,506	47,787	74,389