Basis of presentation and significant accounting policies - Other current assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Total other current assets
Prepaid expenses and other	$ 8,958	$ 6,228
Inventory	8,590	8,063
Total other current assets	17,548	14,291
Total other current liabilities
Accrued interest payable	24,354	24,152
Purchased oil payable	11,971	17,213
Accrued compensation and benefits	9,122	25,947
Lease operating expense payable	8,814	10,572
Other accrued liabilities	20,526	16,331
Total other current liabilities	$ 74,787	$ 94,215