Fair value measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross	$ 6,811	$ 29,622
Derivative liabilities, gross	(22,896)	(4,940)
Derivative assets (liabilities), net	(16,085)	24,682
Derivative assets, amount offset	(5,864)	(3,398)
Derivative liabilities, amounts offset	5,864	3,398
Derivative assets, net	947	26,224
Derivative liabilities, net	(17,032)	(1,542)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net	(16,085)	24,682
Derivative assets, amount offset	(5,864)	(3,398)
Derivative liabilities, amounts offset	5,864	3,398
Derivative assets amount offset (liabilities), net	0	0
Derivative assets, net	947	26,224
Derivative liabilities, net	(17,032)	(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	6,576	29,370
Derivative liabilities, gross	(22,895)	(4,718)
Derivative assets (liabilities), net	(16,319)	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	235	252
Derivative liabilities, gross	(1)	(222)
Derivative assets (liabilities), net	$ 234	$ 30

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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