Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | $ 166,794 | $ 198,805 |
Derivative asset, noncurrent | 63,414 | 77,443 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, noncurrent | (234) | 0 |
Recurring | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Net derivative position | 229,974 | 276,248 |
Recurring | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 160,789 | 194,940 |
Derivative asset, noncurrent | 62,443 | 80,302 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 0 | 0 |
Derivative liability, noncurrent | 0 | 0 |
Recurring | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (7,736) | (9,301) |
Derivative asset, noncurrent | (747) | (4,877) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 0 | 0 |
Derivative liability, noncurrent | 0 | 0 |
Recurring | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 14,401 | 13,166 |
Derivative asset, noncurrent | 2,050 | 2,459 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 0 | 0 |
Derivative liability, noncurrent | 3,345 | 0 |
Recurring | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (660) | 0 |
Derivative asset, noncurrent | (332) | (441) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 0 | 0 |
Derivative liability, noncurrent | (3,579) | 0 |
Recurring | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | 0 | 0 |
Recurring | Current Assets | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (7,736) | (9,301) |
Recurring | Current Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | 0 | 0 |
Recurring | Current Assets | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (660) | 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 | (747) | (4,877) |
Recurring | Noncurrent Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (3,345) | 0 |
Recurring | Noncurrent Assets | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (332) | (441) |
Recurring | Current Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 0 | 0 |
Recurring | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 7,736 | 9,301 |
Recurring | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 0 | 0 |
Recurring | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 660 | 0 |
Recurring | Noncurrent Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 0 | 0 |
Recurring | Noncurrent Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 747 | 4,877 |
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 3,345 | 0 |
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 332 | 441 |
Recurring | Fair value | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Net derivative position | 229,974 | 276,248 |
Recurring | Fair value | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 160,789 | 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 | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 14,401 | 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 | 62,443 | 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 | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 5,395 | 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 | 0 | 0 |
Recurring | Fair value | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (7,736) | (9,301) |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (660) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (747) | (4,877) |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 0 |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (3,911) | (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 | 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 | 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 | 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 | 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 | 243,028 | 290,867 |
Recurring | Level 2 | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 160,789 | 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 | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 14,401 | 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 | 62,443 | 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 | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 5,395 | 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 | 0 | 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 | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 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 | 0 | 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 | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | 0 | 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 | (13,054) | (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 | 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 | 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 | (7,736) | (9,301) |
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 | (660) | 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 | (747) | (4,877) |
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 | $ (3,911) | $ (441) |