Fair value measurements - Summary of changes in assets (liabilities) classified as Level 3 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Changes in assets classified as Level 3 measurements
Change in net present value of deferred premiums for derivatives			$ 141	$ 170
Deferred Premiums
Changes in assets classified as Level 3 measurements
Balance of Level 3 at beginning of period	$ (12,087)	$ (9,025)	(9,285)	(12,684)
Change in net present value of deferred premiums for derivatives	(53)	(50)	(141)	(170)
Purchases	(437)	(3,800)	(5,821)	(3,800)
Settlements	1,248	1,820	3,918	5,599
Balance of Level 3 at end of period	$ (11,329)	$ (11,055)	$ (11,329)	$ (11,055)