Fair value measurements (Recurring) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	$ 16,100	$ 29,622
Derivative liabilities, gross	(6,635)	(4,940)
Derivative assets (liabilities), net	9,465	24,682
Derivative assets, amount offset	(3,543)	(3,398)
Derivative liabilities, amounts offset	3,543	3,398
Derivative assets, net	12,557	26,224
Derivative liabilities, net	(3,092)	(1,542)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net	9,465	24,682
Derivative assets, amount offset	(3,543)	(3,398)
Derivative liabilities, amounts offset	3,543	3,398
Derivative assets amount offset (liabilities), net	0	0
Derivative assets, net	12,557	26,224
Derivative liabilities, net	(3,092)	(1,542)
Recurring Fair Value Measurements | Significant other observable inputs (Level 2)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	14,006	29,370
Derivative liabilities, gross	(6,635)	(4,718)
Derivative assets (liabilities), net	7,371	24,652
Recurring Fair Value Measurements | Significant unobservable inputs (Level 3)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	2,094	252
Derivative liabilities, gross	0	(222)
Derivative assets (liabilities), net	$ 2,094	$ 30