Fair value measurements - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment expense	$ 26,250,000	$ 0
Acquisitions of oil and natural gas properties	22,876,000	0
Nonrecurring | Level 2 | Line-Fill and Other Inventories
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment expense	1,300,000	0
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Acquisitions of oil and natural gas properties	0	0
Nonrecurring | Level 3 | Long-Lived Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment expense	$ 8,200,000	$ 0