Fair value measurements (Recurring) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	$ 53,121	$ 6,811
Derivative liabilities, gross	0	(22,896)
Derivative assets (liabilities), net	53,121	(16,085)
Derivative assets, amount offset	0	(5,864)
Derivative liabilities, amounts offset	0	5,864
Derivative assets, net	53,121	947
Derivative liabilities, net	0	(17,032)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net	53,121	(16,085)
Derivative assets, amount offset	0	(5,864)
Derivative liabilities, amounts offset	0	5,864
Derivative assets amount offset (liabilities), net	0	0
Derivative assets, net	53,121	947
Derivative liabilities, net	0	(17,032)
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	53,016	6,576
Derivative liabilities, gross	0	(22,895)
Derivative assets (liabilities), net	53,016	(16,319)
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	105	235
Derivative liabilities, gross	0	(1)
Derivative assets (liabilities), net	$ 105	$ 234