Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015 |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | $ 64,484 | $ 198,805 |
Derivative asset, noncurrent | 21,872 | 77,443 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (1,628) | 0 |
Derivative liability, noncurrent | (3,101) | 0 |
Recurring | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Net derivative position | 81,627 | 276,248 |
Recurring | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 67,096 | 194,940 |
Derivative asset, noncurrent | 22,124 | 80,302 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 147 | 0 |
Derivative liability, noncurrent | (2,090) | 0 |
Recurring | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (3,595) | (9,301) |
Derivative asset, noncurrent | (252) | (4,877) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (754) | 0 |
Derivative liability, noncurrent | 0 | 0 |
Recurring | Natural Gas Liquids | NGL Derivatives [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 181 | |
Derivative asset, noncurrent | 0 | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 0 | |
Derivative liability, noncurrent | (204) | |
Recurring | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 1,409 | 13,166 |
Derivative asset, noncurrent | 0 | 2,459 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 644 | 0 |
Derivative liability, noncurrent | 2,324 | 0 |
Recurring | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (607) | 0 |
Derivative asset, noncurrent | 0 | (441) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (1,665) | 0 |
Derivative liability, noncurrent | (3,131) | 0 |
Recurring | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (281) | 0 |
Recurring | Current Assets | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (3,595) | (9,301) |
Recurring | Current Assets | Natural Gas Liquids | NGL Derivatives [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (181) | |
Recurring | Current Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (1,282) | 0 |
Recurring | Current Assets | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (607) | 0 |
Recurring | Noncurrent Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | 170 | 0 |
Recurring | Noncurrent Assets | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (252) | (4,877) |
Recurring | Noncurrent Assets | Natural Gas Liquids | NGL Derivatives [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | 72 | |
Recurring | Noncurrent Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (2,602) | 0 |
Recurring | Noncurrent Assets | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | 0 | (441) |
Recurring | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 281 | 0 |
Recurring | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 3,595 | 9,301 |
Recurring | Current Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 181 | |
Recurring | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 1,282 | 0 |
Recurring | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 607 | 0 |
Recurring | Noncurrent Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | (170) | 0 |
Recurring | Noncurrent Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 252 | 4,877 |
Recurring | Noncurrent Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | (72) | |
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 2,602 | 0 |
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 0 | 441 |
Recurring | Fair value | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Net derivative position | 81,627 | 276,248 |
Recurring | Fair value | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 67,377 | 194,940 |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 362 | |
Recurring | Fair value | Current Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 2,691 | 13,166 |
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 | 21,954 | 80,302 |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | (72) | |
Recurring | Fair value | Noncurrent Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 2,602 | 2,459 |
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 | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (134) | 0 |
Recurring | Fair value | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (4,349) | (9,301) |
Recurring | Fair value | Current Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (181) | |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (638) | 0 |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (2,272) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (1,920) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (252) | (4,877) |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (132) | |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (278) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (3,131) | (441) |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 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 | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 1 | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 1 | Current Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | |
Recurring | Level 1 | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 1 | 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 [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 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 | 91,631 | 290,867 |
Recurring | Level 2 | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 67,377 | 194,940 |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 362 | |
Recurring | Level 2 | Current Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 2,691 | 13,166 |
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 | 21,954 | 80,302 |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | (72) | |
Recurring | Level 2 | Noncurrent Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 2,602 | 2,459 |
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 | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (134) | 0 |
Recurring | Level 2 | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 2 | Current Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (181) | |
Recurring | Level 2 | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (638) | 0 |
Recurring | Level 2 | 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 | (1,920) | 0 |
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 [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (132) | |
Recurring | Level 2 | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (278) | 0 |
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 | (10,004) | (14,619) |
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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 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 [Member] | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 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 | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 3 | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (4,349) | (9,301) |
Recurring | Level 3 | Current Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | |
Recurring | Level 3 | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Level 3 | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (2,272) | 0 |
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 | (252) | (4,877) |
Recurring | Level 3 | Noncurrent Liabilities | Natural Gas Liquids | NGL Derivatives [Member] | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 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 | $ (3,131) | $ (441) |