Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Derivative asset, current	$ 15,442	$ 6,892
Derivative asset, noncurrent	4,541	3,413
Liabilities:
Derivative liability, current	(17,628)	(22,950)
Derivative liability, noncurrent	(325)	(384)
Recurring
Liabilities:
Net derivative position	2,030	(13,029)
Recurring | Crude Oil | Commodity derivatives
Assets:
Derivative asset, current	7,295	3,706
Derivative asset, noncurrent	8,414	5,526
Liabilities:
Derivative liability, current	(10,300)	(8,756)
Derivative liability, noncurrent	1,361	3,698
Recurring | Crude Oil | Deferred Premiums
Assets:
Derivative asset, current	(5,930)	(87)
Derivative asset, noncurrent	(8,001)	(2,113)
Liabilities:
Derivative liability, current	(12,368)	(18,115)
Derivative liability, noncurrent	(1,468)	(5,016)
Recurring | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative asset, current	101	0
Derivative asset, noncurrent	0	0
Liabilities:
Derivative liability, current	470	0
Derivative liability, noncurrent	0	0
Recurring | Natural Gas | Commodity derivatives
Assets:
Derivative asset, current	16,153	5,729
Derivative asset, noncurrent	4,128	0
Liabilities:
Derivative liability, current	4,918	4,817
Derivative liability, noncurrent	(218)	934
Recurring | Natural Gas | Deferred Premiums
Assets:
Derivative asset, current	(2,177)	(2,456)
Derivative asset, noncurrent	0	0
Liabilities:
Derivative liability, current	(348)	(896)
Derivative liability, noncurrent	0	0
Recurring | Current Assets | Crude Oil | Commodity derivatives
Assets:
Amounts offset	(6,937)	(3,721)
Recurring | Current Assets | Crude Oil | Deferred Premiums
Assets:
Amounts offset	(5,930)	(87)
Recurring | Current Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Amounts offset	(792)	0
Recurring | Current Assets | Natural Gas | Commodity derivatives
Assets:
Amounts offset	(4,918)	(4,817)
Recurring | Current Assets | Natural Gas | Deferred Premiums
Assets:
Amounts offset	(2,177)	(2,456)
Recurring | Noncurrent Assets | Crude Oil | Commodity derivatives
Assets:
Amounts offset	(5,581)	(6,087)
Recurring | Noncurrent Assets | Crude Oil | Deferred Premiums
Assets:
Amounts offset	(8,001)	(2,113)
Recurring | Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Amounts offset	0	0
Recurring | Noncurrent Assets | Natural Gas | Commodity derivatives
Assets:
Amounts offset	0	(934)
Recurring | Noncurrent Assets | Natural Gas | Deferred Premiums
Assets:
Amounts offset	0	0
Recurring | Other current liabilities | Crude Oil | Commodity derivatives
Liabilities:
Amounts offset	6,937	3,721
Recurring | Other current liabilities | Crude Oil | Deferred Premiums
Liabilities:
Amounts offset	5,930	87
Recurring | Other current liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Amounts offset	792	0
Recurring | Other current liabilities | Natural Gas | Commodity derivatives
Liabilities:
Amounts offset	4,918	4,817
Recurring | Other current liabilities | Natural Gas | Deferred Premiums
Liabilities:
Amounts offset	2,177	2,456
Recurring | Noncurrent Liabilities | Crude Oil | Commodity derivatives
Liabilities:
Amounts offset	5,581	6,087
Recurring | Noncurrent Liabilities | Crude Oil | Deferred Premiums
Liabilities:
Amounts offset	8,001	2,113
Recurring | Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Amounts offset	0	0
Recurring | Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities:
Amounts offset	0	934
Recurring | Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities:
Amounts offset	0	0
Recurring | Fair value
Liabilities:
Net derivative position	2,030	(13,029)
Recurring | Fair value | Current Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	14,232	7,427
Recurring | Fair value | Current Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Fair value | Current Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	893	0
Recurring | Fair value | Current Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	21,071	10,546
Recurring | Fair value | Current Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	13,995	11,613
Recurring | Fair value | Noncurrent Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	4,128	934
Recurring | Fair value | Noncurrent Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Fair value | Other current liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(17,237)	(12,477)
Recurring | Fair value | Other current liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(18,298)	(18,202)
Recurring | Fair value | Other current liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(322)	0
Recurring | Fair value | Other current liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Fair value | Other current liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(2,525)	(3,352)
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(4,220)	(2,389)
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(9,469)	(7,129)
Recurring | Fair value | Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(218)	0
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1
Liabilities:
Net derivative position	0	0
Recurring | Level 1 | Current Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 1 | Other current liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2
Liabilities:
Net derivative position	32,322	15,654
Recurring | Level 2 | Current Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	14,232	7,427
Recurring | Level 2 | Current Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 2 | Current Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	893	0
Recurring | Level 2 | Current Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	21,071	10,546
Recurring | Level 2 | Current Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	13,995	11,613
Recurring | Level 2 | Noncurrent Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	4,128	934
Recurring | Level 2 | Noncurrent Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 2 | Other current liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(17,237)	(12,477)
Recurring | Level 2 | Other current liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2 | Other current liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(322)	0
Recurring | Level 2 | Other current liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2 | Other current liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(4,220)	(2,389)
Recurring | Level 2 | Noncurrent Liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	(218)	0
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3
Liabilities:
Net derivative position	(30,292)	(28,683)
Recurring | Level 3 | Current Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Crude Oil | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Crude Oil | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas | Commodity derivatives
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas | Deferred Premiums
Assets:
Derivative assets before netting	0	0
Recurring | Level 3 | Other current liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(18,298)	(18,202)
Recurring | Level 3 | Other current liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(2,525)	(3,352)
Recurring | Level 3 | Noncurrent Liabilities | Crude Oil | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Crude Oil | Deferred Premiums
Liabilities:
Derivative liabilities before netting	(9,469)	(7,129)
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities:
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities:
Derivative liabilities before netting	$ 0	$ 0