Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	$ 42,985	$ 20,947
Derivative asset, noncurrent	12,807	8,718
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	(653)	(20,993)
Derivative liability, noncurrent	0	(5,694)
Recurring
Liabilities, Fair Value Disclosure [Abstract]
Net derivative position	55,139	2,978
Recurring | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	46,169	22,527
Derivative asset, noncurrent	14,712	8,718
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	119	(9,789)
Derivative liability, noncurrent	0	(4,552)
Recurring | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	(4,637)	(1,580)
Derivative asset, noncurrent	(3,068)	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	0	(1,989)
Derivative liability, noncurrent	0	0
Recurring | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	(568)	0
Derivative asset, noncurrent	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	0	(2,803)
Derivative liability, noncurrent	0	0
Recurring | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	2,875	0
Derivative asset, noncurrent	2,822	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	1,564	(3,369)
Derivative liability, noncurrent	0	1,244
Recurring | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative asset, current	(854)	0
Derivative asset, noncurrent	(1,659)	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability, current	(2,336)	(3,043)
Derivative liability, noncurrent	0	(2,386)
Recurring | Current Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(323)	0
Recurring | Current Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(4,637)	(1,580)
Recurring | Current Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(678)	0
Recurring | Current Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(1,577)	(270)
Recurring | Current Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(854)	0
Recurring | Noncurrent Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	0	0
Recurring | Noncurrent Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(3,068)	0
Recurring | Noncurrent Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	0	0
Recurring | Noncurrent Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Amounts offset	0	(1,377)
Recurring | Noncurrent Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Amounts offset	(1,659)	0
Recurring | Other current liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	323	0
Recurring | Other current liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	4,637	1,580
Recurring | Other current liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	678	0
Recurring | Other current liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	1,577	270
Recurring | Other current liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	854	0
Recurring | Noncurrent Liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	0	0
Recurring | Noncurrent Liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	3,068	0
Recurring | Noncurrent Liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	0	0
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	0	1,377
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Amounts offset	1,659	0
Recurring | Fair value
Liabilities, Fair Value Disclosure [Abstract]
Net derivative position	55,139	2,978
Recurring | Fair value | Current Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	46,492	22,527
Recurring | Fair value | Current Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Fair value | Current Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	110	0
Recurring | Fair value | Current Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	4,452	270
Recurring | Fair value | Current Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	14,712	8,718
Recurring | Fair value | Noncurrent Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Fair value | Noncurrent Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	2,822	1,377
Recurring | Fair value | Noncurrent Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Fair value | Other current liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(204)	(9,789)
Recurring | Fair value | Other current liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(4,637)	(3,569)
Recurring | Fair value | Other current liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(678)	(2,803)
Recurring | Fair value | Other current liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(13)	(3,639)
Recurring | Fair value | Other current liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(3,190)	(3,043)
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	(4,552)
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(3,068)	0
Recurring | Fair value | Noncurrent Liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	(133)
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(1,659)	(2,386)
Recurring | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Net derivative position	0	0
Recurring | Level 1 | Current Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Current Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Noncurrent Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 1 | Other current liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Other current liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 1 | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Net derivative position	67,693	11,976
Recurring | Level 2 | Current Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	46,492	22,527
Recurring | Level 2 | Current Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 2 | Current Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	110	0
Recurring | Level 2 | Current Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	4,452	270
Recurring | Level 2 | Current Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	14,712	8,718
Recurring | Level 2 | Noncurrent Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 2 | Noncurrent Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	2,822	1,377
Recurring | Level 2 | Noncurrent Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 2 | Other current liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(204)	(9,789)
Recurring | Level 2 | Other current liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 2 | Other current liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(678)	(2,803)
Recurring | Level 2 | Other current liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(13)	(3,639)
Recurring | Level 2 | Other current liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	(4,552)
Recurring | Level 2 | Noncurrent Liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	(133)
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Net derivative position	(12,554)	(8,998)
Recurring | Level 3 | Current Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Current Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Crude Oil | Oil derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Crude Oil | Oil deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas Liquids | NGL derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas | Natural gas derivatives
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Noncurrent Assets | Natural Gas | Natural gas deferred premiums
Assets, Fair Value Disclosure [Abstract]
Derivative assets before netting	0	0
Recurring | Level 3 | Other current liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(4,637)	(3,569)
Recurring | Level 3 | Other current liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Other current liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(3,190)	(3,043)
Recurring | Level 3 | Noncurrent Liabilities | Crude Oil | Oil derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Crude Oil | Oil deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	(3,068)	0
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas Liquids | NGL derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas | Natural gas derivatives
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	0	0
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities before netting	$ (1,659)	$ (2,386)