Disaggregation of oil, gas, and NGL production revenue (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		$ 406,854	[1]	$ 402,558	[1]	$ 747,330	[2]	$ 785,444	[2]
Revenue, Remaining Performance Obligation, Amount		0				0
Oil production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		304,144	[1]	266,150	[1]	543,205	[2]	527,210	[2]
Gas production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		65,224	[1]	84,064	[1]	130,337	[2]	163,173	[2]
NGL production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		37,486	[1]	52,344	[1]	73,788	[2]	95,061	[2]
Permian [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		304,853	[1]	259,499	[1]	545,713	[2]	490,294	[2]
Permian [Member] | Oil production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		288,447		227,636		513,694		433,430
Permian [Member] | Gas production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		16,449		31,734		32,041		56,611
Permian [Member] | NGL production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		(43)		129		(22)		253
South Texas & Gulf Coast [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		102,001	[1]	123,829	[1]	201,617	[2]	237,915	[2]
South Texas & Gulf Coast [Member] | Oil production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		15,697		19,346		29,511		38,929
South Texas & Gulf Coast [Member] | Gas production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		48,775		52,235		98,296		104,968
South Texas & Gulf Coast [Member] | NGL production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		37,529		52,248		73,810		94,018
Rocky Mountain [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		0	[1]	19,230	[1]	0	[2]	57,235	[2]
Rocky Mountain [Member] | Oil production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		0		19,168		0		54,851
Rocky Mountain [Member] | Gas production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		0		95		0		1,594
Rocky Mountain [Member] | NGL production revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax		$ 0		$ (33)		$ 0		$ 790
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Segment Reporting Information [Line Items]
Relative percentage	[1]	100.00%		100.00%		100.00%		100.00%
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member] | Permian [Member]
Segment Reporting Information [Line Items]
Relative percentage		75.00%	[1]	64.00%	[1]	73.00%	[2]	63.00%	[2]
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member] | South Texas & Gulf Coast [Member]
Segment Reporting Information [Line Items]
Relative percentage		25.00%	[1]	31.00%	[1]	27.00%	[2]	30.00%	[2]
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member] | Rocky Mountain [Member]
Segment Reporting Information [Line Items]
Relative percentage		0.00%	[1]	5.00%	[1]	0.00%	[2]	7.00%	[2]

[1] Note: Amounts may not calculate due to rounding.

[2] Note: Amounts may not calculate due to rounding.