Supplemental oil, NGL and natural gas disclosures (unaudited) - Net proved oil, NGL and natural gas reserves - (unaudited) - Narrative (Details) Boe in Thousands	12 Months Ended
	Dec. 31, 2018 Boe reserves_stream location	Dec. 31, 2017 Boe reserves_stream location	Dec. 31, 2016 Boe reserves_stream location
Net proved oil and natural gas reserves
Percentage of proved reserves estimated by independent reserve engineers (percent)	100.00%	100.00%	100.00%
Number of reportable reserves streams | reserves_stream	3	3	3
Revisions of previous estimates (MBOE)	2,173	35,351	34,082
Development wells drilled, net productive | location	8	10	4
Development wells, scheduled to be drilled in the next twelve months | location	2	8	7
Extensions, discoveries and other additions (MBOE)	44,069	34,921	24,940
Performance, Pricing and Other Increases
Net proved oil and natural gas reserves
Revisions of previous estimates (MBOE)	11,364
Performance, Pricing and Other Changes
Net proved oil and natural gas reserves
Revisions of previous estimates (MBOE)	7,045	16,916	26,049
Reinterpretation of Undeveloped Locations
Net proved oil and natural gas reserves
Revisions of previous estimates (MBOE)	6,492	18,435	10,325
Removed due to derecognition of certain proved undeveloped locations
Net proved oil and natural gas reserves
Revisions of previous estimates (MBOE)			2,292
Drilling of New Wells
Net proved oil and natural gas reserves
Extensions, discoveries and other additions (MBOE)	25,617	18,985	13,302
Horizontal Proved Undeveloped Properties
Net proved oil and natural gas reserves
Extensions, discoveries and other additions (MBOE)	18,452	15,936	11,638