Fair value measurements (Recurring fair value measurements) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	$ 29,622	$ 1,332
Derivative liabilities, gross	(4,940)	(14,458)
Derivative assets (liabilities), net	24,682	(13,126)
Derivative assets, amount offset	(3,398)	(1,332)
Derivative liabilities, amounts offset	3,398	1,332
Derivative assets, net	26,224	0
Derivative liabilities, net	(1,542)	(13,126)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net	24,682	(13,126)
Derivative assets, amount offset	(3,398)	(1,332)
Derivative liabilities, amounts offset	3,398	1,332
Derivative assets amount offset (liabilities), net	0	0
Derivative assets, net	26,224	0
Derivative liabilities, net	(1,542)	(13,126)
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	29,370	1,332
Derivative liabilities, gross	(4,718)	(14,163)
Derivative assets (liabilities), net	24,652	(12,831)
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	252	0
Derivative liabilities, gross	(222)	(295)
Derivative assets (liabilities), net	$ 30	$ (295)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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