Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands | Jun. 30, 2016 | Dec. 31, 2015 |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | $ 88,965 | $ 198,805 |
Derivative asset, noncurrent | 32,680 | 77,443 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (1,480) | 0 |
Derivative liability, noncurrent | (3,740) | 0 |
Recurring | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Net derivative position | 116,425 | 276,248 |
Recurring | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 92,542 | 194,940 |
Derivative asset, noncurrent | 33,181 | 80,302 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 515 | 0 |
Derivative liability, noncurrent | (1,820) | 0 |
Recurring | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (5,467) | (9,301) |
Derivative asset, noncurrent | (501) | (4,877) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (1,317) | 0 |
Derivative liability, noncurrent | 0 | 0 |
Recurring | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | 2,293 | 13,166 |
Derivative asset, noncurrent | 0 | 2,459 |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | 426 | 0 |
Derivative liability, noncurrent | 1,950 | 0 |
Recurring | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative asset, current | (403) | 0 |
Derivative asset, noncurrent | 0 | (441) |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liability, current | (1,104) | 0 |
Derivative liability, noncurrent | (3,870) | 0 |
Recurring | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (515) | 0 |
Recurring | Current Assets | Crude Oil | Oil deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (5,467) | (9,301) |
Recurring | Current Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (1,165) | 0 |
Recurring | Current Assets | Natural Gas | Natural gas deferred premiums | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (403) | 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 | (501) | (4,877) |
Recurring | Noncurrent Assets | Natural Gas | Natural gas derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Amounts offset | (2,724) | 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 | 515 | 0 |
Recurring | Current Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 5,467 | 9,301 |
Recurring | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 1,165 | 0 |
Recurring | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 403 | 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 | 501 | 4,877 |
Recurring | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Amount offset | 2,724 | 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 | 116,425 | 276,248 |
Recurring | Fair value | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 93,057 | 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 | 3,458 | 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 | 33,181 | 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 | 2,724 | 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 | (6,784) | (9,301) |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (739) | 0 |
Recurring | Fair value | Current Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (1,507) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (1,820) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Crude Oil | Oil deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (501) | (4,877) |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas derivatives | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (774) | 0 |
Recurring | Fair value | Noncurrent Liabilities | Natural Gas | Natural gas deferred premiums | | |
Liabilities, Fair Value Disclosure [Abstract] | | |
Derivative liabilities before netting | (3,870) | (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 | 129,087 | 290,867 |
Recurring | Level 2 | Current Assets | Crude Oil | Oil derivatives | | |
Assets, Fair Value Disclosure [Abstract] | | |
Derivative assets before netting | 93,057 | 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 | 3,458 | 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 | 33,181 | 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 | 2,724 | 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 | (739) | 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 | (1,820) | 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 | (774) | 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 | (12,662) | (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 | (6,784) | (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 | (1,507) | 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 | (501) | (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 | $ (3,870) | $ (441) |