Fair value measurements (Derivatives Offset in Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 17,687	$ 6,811
Derivative assets, amount offset	(500)	(5,864)
Derivative assets, net	17,187	947
Derivative assets, not offset	0	0
Credit facility balance available to offset net derivative assets	(17,187)	(947)
Derivative asset, net	0	0
Derivative liabilities, gross	(500)	(22,896)
Derivative liabilities, amounts offset	500	5,864
Derivative liabilities, net	0	(17,032)
Derivative liabilities, not offset	0	0
Derivative liabilities, outstanding under credit facility	0	947
Derivative liability, net	0	(16,085)
Derivative asset (liability), gross	17,187	(16,085)
Derivative asset (liability), amount offset	0	0
Derivative asset (liability), net	17,187	(16,085)
Derivative asset (liability) not offset	0	0
Derivative asset (liability). outstanding under credit facility	(17,187)	0
Derivative asset (liability), net	$ 0	$ (16,085)