Basis of presentation and significant accounting policies - Other current assets and liabilities (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Total other current assets
Inventory	[1]	$ 6,986	$ 6,974
Prepaid expenses and other		7,459	7,600
Total other current assets		14,445	14,574
Total other current liabilities
Accrued interest payable		24,161	24,208
Costs of purchased oil payable		15,629	12,189
Lease operating expense payable		11,570	13,205
Accrued compensation and benefits		8,720	14,342
Capital contribution payable to equity method investee	[2]	0	27,583
Other accrued liabilities		12,440	14,695
Total other current liabilities		$ 72,520	$ 106,222

[1] See Note 2.g for discussion of inventory held by the Company.

[2] (1)See Notes 14 and 15 for additional discussion regarding our equity method investee.