Segments - Selected financial information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Oil, NGL and natural gas sales									$ 621,507	$ 426,485	$ 431,734
Midstream service revenues									10,517	8,342	6,548
Sales of purchased oil									190,138	162,551	168,358
Total revenues	$ 240,337	$ 205,818	$ 187,001	$ 189,006	$ 184,314	$ 159,734	$ 146,773	$ 106,557	822,162	597,378	606,640
Costs and expenses:
Lease operating expenses, including production and ad valorem tax									112,851	103,913	141,233
Midstream service expenses									4,099	4,077	5,846
Costs of purchased oil									195,908	169,536	174,338
General and administrative									96,312	91,756	90,425
Depletion, depreciation and amortization									158,389	148,339	277,724
Impairment expense									0	162,027	2,374,888
Restructuring charges and other operating expenses									4,931	5,692	13,700
Operating income (loss)	85,833	$ 60,452	$ 52,061	$ 51,326	45,460	$ 25,492	$ 17,874	$ (176,788)	249,672	(87,962)	(2,471,514)
Other financial information:
Income from equity method investee (see Note 4.a)									8,485	9,403	6,799
Interest expense									(89,377)	(93,298)	(103,219)
Loss on early redemption of debt									(23,761)	0	(31,537)
Gain on sale of investment in equity method investee (see Note 4.a)									405,906	0	0
Capital expenditures									(563,914)	(373,530)	(632,601)
Gross property and equipment	6,482,103				6,172,024				6,482,103	6,172,024	5,645,976
Operating Segments | Exploration and production
Revenues:
Oil, NGL and natural gas sales									623,401	427,231	432,711
Midstream service revenues									0	0	0
Sales of purchased oil									0	0	0
Total revenues									623,401	427,231	432,711
Costs and expenses:
Lease operating expenses, including production and ad valorem tax									126,779	115,496	151,918
Midstream service expenses									0	0	0
Costs of purchased oil									0	0	0
General and administrative									88,113	83,901	82,251
Depletion, depreciation and amortization									148,828	139,407	269,631
Impairment expense										162,027	2,372,296
Restructuring charges and other operating expenses									4,707	5,483	12,522
Operating income (loss)									254,974	(79,083)	(2,455,907)
Other financial information:
Income from equity method investee (see Note 4.a)									0	0	0
Interest expense									(83,758)	(87,485)	(98,040)
Loss on early redemption of debt									(22,225)		(30,056)
Gain on sale of investment in equity method investee (see Note 4.a)									0
Capital expenditures									(543,027)	(368,290)	(597,086)
Gross property and equipment	6,321,725				5,780,137				6,321,725	5,780,137	5,302,716
Operating Segments | Midstream and marketing
Revenues:
Oil, NGL and natural gas sales									3,301	1,141	1,692
Midstream service revenues									72,643	49,971	27,965
Sales of purchased oil									190,138	162,551	168,358
Total revenues									266,082	213,663	198,015
Costs and expenses:
Lease operating expenses, including production and ad valorem tax									0	0	0
Midstream service expenses									49,017	29,693	17,557
Costs of purchased oil									195,908	169,536	174,338
General and administrative									8,199	7,855	8,174
Depletion, depreciation and amortization									9,561	8,932	8,093
Impairment expense										0	2,592
Restructuring charges and other operating expenses									224	209	1,178
Operating income (loss)									3,173	(2,562)	(13,917)
Other financial information:
Income from equity method investee (see Note 4.a)									8,485	9,403	6,799
Interest expense									(5,619)	(5,813)	(5,179)
Loss on early redemption of debt									(1,536)		(1,481)
Gain on sale of investment in equity method investee (see Note 4.a)									405,906
Capital expenditures									(20,887)	(5,240)	(35,515)
Gross property and equipment	177,093				400,127				177,093	400,127	345,183
Eliminations
Revenues:
Oil, NGL and natural gas sales									(5,195)	(1,887)	(2,669)
Midstream service revenues									(62,126)	(41,629)	(21,417)
Sales of purchased oil									0	0	0
Total revenues									(67,321)	(43,516)	(24,086)
Costs and expenses:
Lease operating expenses, including production and ad valorem tax									(13,928)	(11,583)	(10,685)
Midstream service expenses									(44,918)	(25,616)	(11,711)
Costs of purchased oil									0	0	0
General and administrative									0	0	0
Depletion, depreciation and amortization									0	0	0
Impairment expense										0	0
Restructuring charges and other operating expenses									0	0	0
Operating income (loss)									(8,475)	(6,317)	(1,690)
Other financial information:
Income from equity method investee (see Note 4.a)									0	0	0
Interest expense									0	0	0
Loss on early redemption of debt									0		0
Gain on sale of investment in equity method investee (see Note 4.a)									0
Capital expenditures									0	0	0
Gross property and equipment	$ (16,715)				$ (8,240)				$ (16,715)	$ (8,240)	$ (1,923)