Fair value measurements - Fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Net fair value presented on the consolidated balance sheets	$ 39,835	$ 6,892
Net fair value presented on the consolidated balance sheets	11,030	3,413
Liabilities
Net fair value presented on the consolidated balance sheets	(7,359)	(22,950)
Net fair value presented on the consolidated balance sheets	0	(384)
Net derivative asset (liability) positions	43,506	(13,029)
Level 1
Liabilities
Net derivative asset (liability) positions	0	0
Level 2
Liabilities
Net derivative asset (liability) positions	60,071	15,654
Level 3
Liabilities
Net derivative asset (liability) positions	(16,565)	(28,683)
Oil derivatives | Commodity derivatives
Assets
Net fair value presented on the consolidated balance sheets	36,518	3,706
Net fair value presented on the consolidated balance sheets	10,626	5,526
Liabilities
Net fair value presented on the consolidated balance sheets	(1,152)	(8,756)
Net fair value presented on the consolidated balance sheets	0	3,698
Oil derivatives | Natural Gas Liquids
Assets
Net fair value presented on the consolidated balance sheets		0
Net fair value presented on the consolidated balance sheets		0
Oil derivatives | Deferred Premiums
Assets
Net fair value presented on the consolidated balance sheets	(14,381)	(87)
Net fair value presented on the consolidated balance sheets	0	(2,113)
Liabilities
Net fair value presented on the consolidated balance sheets	(2,184)	(18,115)
Net fair value presented on the consolidated balance sheets	0	(5,016)
Natural Gas Liquids | Commodity derivatives
Assets
Net fair value presented on the consolidated balance sheets	1,974
Net fair value presented on the consolidated balance sheets	1,024
Liabilities
Net fair value presented on the consolidated balance sheets	0	0
Net fair value presented on the consolidated balance sheets	0	0
Natural Gas | Commodity derivatives
Assets
Net fair value presented on the consolidated balance sheets	15,724	5,729
Net fair value presented on the consolidated balance sheets	(620)	0
Liabilities
Net fair value presented on the consolidated balance sheets	(4,023)	4,817
Net fair value presented on the consolidated balance sheets	0	934
Natural Gas | Deferred Premiums
Assets
Net fair value presented on the consolidated balance sheets	0	(2,456)
Net fair value presented on the consolidated balance sheets	0	0
Liabilities
Net fair value presented on the consolidated balance sheets	0	(896)
Net fair value presented on the consolidated balance sheets	0	0
Current Assets | Oil derivatives | Commodity derivatives
Assets
Total gross fair value	44,425	7,427
Amounts offset	(7,907)	(3,721)
Current Assets | Oil derivatives | Natural Gas Liquids
Assets
Total gross fair value		0
Amounts offset		0
Current Assets | Oil derivatives | Deferred Premiums
Assets
Total gross fair value	0	0
Amounts offset	(14,381)	(87)
Current Assets | Oil derivatives | Level 1 | Commodity derivatives
Assets
Total gross fair value	0	0
Current Assets | Oil derivatives | Level 1 | Natural Gas Liquids
Assets
Total gross fair value		0
Current Assets | Oil derivatives | Level 1 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Assets | Oil derivatives | Level 2 | Commodity derivatives
Assets
Total gross fair value	44,425	7,427
Current Assets | Oil derivatives | Level 2 | Natural Gas Liquids
Assets
Total gross fair value		0
Current Assets | Oil derivatives | Level 2 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Assets | Oil derivatives | Level 3 | Commodity derivatives
Assets
Total gross fair value	0	0
Current Assets | Oil derivatives | Level 3 | Natural Gas Liquids
Assets
Total gross fair value		0
Current Assets | Oil derivatives | Level 3 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Assets | Natural Gas Liquids | Commodity derivatives
Assets
Total gross fair value	1,974
Amounts offset	0
Current Assets | Natural Gas Liquids | Level 1 | Commodity derivatives
Assets
Total gross fair value	0
Current Assets | Natural Gas Liquids | Level 2 | Commodity derivatives
Assets
Total gross fair value	1,974
Current Assets | Natural Gas Liquids | Level 3 | Commodity derivatives
Assets
Total gross fair value	0
Current Assets | Natural Gas | Commodity derivatives
Assets
Total gross fair value	18,991	10,546
Amounts offset	(3,267)	(4,817)
Current Assets | Natural Gas | Deferred Premiums
Assets
Total gross fair value	0	0
Amounts offset	0	(2,456)
Current Assets | Natural Gas | Level 1 | Commodity derivatives
Assets
Total gross fair value	0	0
Current Assets | Natural Gas | Level 1 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Assets | Natural Gas | Level 2 | Commodity derivatives
Assets
Total gross fair value	18,991	10,546
Current Assets | Natural Gas | Level 2 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Assets | Natural Gas | Level 3 | Commodity derivatives
Assets
Total gross fair value	0	0
Current Assets | Natural Gas | Level 3 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Oil derivatives | Commodity derivatives
Assets
Total gross fair value	10,626	11,613
Amounts offset	0	(6,087)
Noncurrent Assets | Oil derivatives | Natural Gas Liquids
Assets
Total gross fair value		0
Amounts offset		0
Noncurrent Assets | Oil derivatives | Deferred Premiums
Assets
Total gross fair value	0	0
Amounts offset	0	(2,113)
Noncurrent Assets | Oil derivatives | Level 1 | Commodity derivatives
Assets
Total gross fair value	0	0
Noncurrent Assets | Oil derivatives | Level 1 | Natural Gas Liquids
Assets
Total gross fair value		0
Noncurrent Assets | Oil derivatives | Level 1 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Oil derivatives | Level 2 | Commodity derivatives
Assets
Total gross fair value	10,626	11,613
Noncurrent Assets | Oil derivatives | Level 2 | Natural Gas Liquids
Assets
Total gross fair value		0
Noncurrent Assets | Oil derivatives | Level 2 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Oil derivatives | Level 3 | Commodity derivatives
Assets
Total gross fair value	0	0
Noncurrent Assets | Oil derivatives | Level 3 | Natural Gas Liquids
Assets
Total gross fair value		0
Noncurrent Assets | Oil derivatives | Level 3 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Natural Gas Liquids | Commodity derivatives
Assets
Total gross fair value	1,024
Amounts offset	0
Noncurrent Assets | Natural Gas Liquids | Level 1 | Commodity derivatives
Assets
Total gross fair value	0
Noncurrent Assets | Natural Gas Liquids | Level 2 | Commodity derivatives
Assets
Total gross fair value	1,024
Noncurrent Assets | Natural Gas Liquids | Level 3 | Commodity derivatives
Assets
Total gross fair value	0
Noncurrent Assets | Natural Gas | Commodity derivatives
Assets
Total gross fair value	108	934
Amounts offset	(728)	(934)
Noncurrent Assets | Natural Gas | Deferred Premiums
Assets
Total gross fair value	0	0
Amounts offset	0	0
Noncurrent Assets | Natural Gas | Level 1 | Commodity derivatives
Assets
Total gross fair value	0	0
Noncurrent Assets | Natural Gas | Level 1 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Natural Gas | Level 2 | Commodity derivatives
Assets
Total gross fair value	108	934
Noncurrent Assets | Natural Gas | Level 2 | Deferred Premiums
Assets
Total gross fair value	0	0
Noncurrent Assets | Natural Gas | Level 3 | Commodity derivatives
Assets
Total gross fair value	0	0
Noncurrent Assets | Natural Gas | Level 3 | Deferred Premiums
Assets
Total gross fair value	0	0
Current Liabilities | Oil derivatives | Commodity derivatives
Liabilities
Total gross fair value	(9,059)	(12,477)
Amounts offset	7,907	3,721
Current Liabilities | Oil derivatives | Deferred Premiums
Liabilities
Total gross fair value	(16,565)	(18,202)
Amounts offset	14,381	87
Current Liabilities | Oil derivatives | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Oil derivatives | Level 1 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Current Liabilities | Oil derivatives | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	(9,059)	(12,477)
Current Liabilities | Oil derivatives | Level 2 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Current Liabilities | Oil derivatives | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Oil derivatives | Level 3 | Deferred Premiums
Liabilities
Total gross fair value	(16,565)	(18,202)
Current Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities
Total gross fair value	0	0
Amounts offset	0	0
Current Liabilities | Natural Gas Liquids | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas Liquids | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas Liquids | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas | Commodity derivatives
Liabilities
Total gross fair value	(7,290)	0
Amounts offset	3,267	4,817
Current Liabilities | Natural Gas | Deferred Premiums
Liabilities
Total gross fair value	0	(3,352)
Amounts offset	0	2,456
Current Liabilities | Natural Gas | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas | Level 1 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	(7,290)	0
Current Liabilities | Natural Gas | Level 2 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Current Liabilities | Natural Gas | Level 3 | Deferred Premiums
Liabilities
Total gross fair value	0	(3,352)
Noncurrent Liabilities | Oil derivatives | Commodity derivatives
Liabilities
Total gross fair value	0	(2,389)
Amounts offset	0	6,087
Noncurrent Liabilities | Oil derivatives | Deferred Premiums
Liabilities
Total gross fair value	0	(7,129)
Amounts offset	0	2,113
Noncurrent Liabilities | Oil derivatives | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Oil derivatives | Level 1 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Oil derivatives | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	0	(2,389)
Noncurrent Liabilities | Oil derivatives | Level 2 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Oil derivatives | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Oil derivatives | Level 3 | Deferred Premiums
Liabilities
Total gross fair value	0	(7,129)
Noncurrent Liabilities | Natural Gas Liquids | Commodity derivatives
Liabilities
Total gross fair value	0	0
Amounts offset	0	0
Noncurrent Liabilities | Natural Gas Liquids | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas Liquids | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas Liquids | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas | Commodity derivatives
Liabilities
Total gross fair value	(728)	0
Amounts offset	728	934
Noncurrent Liabilities | Natural Gas | Deferred Premiums
Liabilities
Total gross fair value	0	0
Amounts offset	0	0
Noncurrent Liabilities | Natural Gas | Level 1 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas | Level 1 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas | Level 2 | Commodity derivatives
Liabilities
Total gross fair value	(728)	0
Noncurrent Liabilities | Natural Gas | Level 2 | Deferred Premiums
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas | Level 3 | Commodity derivatives
Liabilities
Total gross fair value	0	0
Noncurrent Liabilities | Natural Gas | Level 3 | Deferred Premiums
Liabilities
Total gross fair value	$ 0	$ 0