Fair value measurements (Derivatives Offset in Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 11,841	$ 29,622
Derivative assets, amount offset	(7,096)	(3,398)
Derivative assets	4,745	26,224
Derivative assets, not offset	(887)	(1,542)
Credit facility balance available to offset net derivative assets	(3,858)	0
Derivative asset, net	0	24,682
Derivative liabilities, gross	(21,094)	(4,940)
Derivative liabilities, amounts offset	7,096	3,398
Derivative liabilities	(13,998)	(1,542)
Derivative liabilities, not offset	887	1,542
Derivative liability, net	(13,111)	0
Derivative asset (liability), gross	(9,253)	24,682
Derivative asset (liability), amount offset	0	0
Derivative asset (liability), net	(9,253)	24,682
Derivative asset (liability) not offset	0	0
Derivative asset (liability), net	$ (13,111)	$ 24,682