Fair value measurements (Recurring) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	$ 11,841	$ 29,622
Derivative liabilities, gross	(21,094)	(4,940)
Derivative assets (liabilities), net	(9,253)	24,682
Derivative assets, amount offset	(7,096)	(3,398)
Derivative liabilities, amounts offset	7,096	3,398
Derivative assets, net	4,745	26,224
Derivative liabilities, net	(13,998)	(1,542)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net	(9,253)	24,682
Derivative assets, amount offset	(7,096)	(3,398)
Derivative liabilities, amounts offset	7,096	3,398
Derivative assets amount offset (liabilities), net	0	0
Derivative assets, net	4,745	26,224
Derivative liabilities, net	(13,998)	(1,542)
Recurring Fair Value Measurements | Significant Other Observable Inputs
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	11,109	29,370
Derivative liabilities, gross	(20,949)	(4,718)
Derivative assets (liabilities), net	(9,840)	24,652
Recurring Fair Value Measurements | Significant Unobservable Inputs
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	732	252
Derivative liabilities, gross	(145)	(222)
Derivative assets (liabilities), net	$ 587	$ 30