Fair value measurements (Derivatives Offset in Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 16,100	$ 29,622
Derivative assets, amount offset	(3,543)	(3,398)
Derivative assets, net	12,557	26,224
Derivative assets, not offset	(1,757)	(1,542)
Credit facility balance available to offset net derivative assets	(10,800)	0
Derivative asset, net	0	24,682
Derivative liabilities, gross	(6,635)	(4,940)
Derivative liabilities, amounts offset	3,543	3,398
Derivative liabilities	(3,092)	(1,542)
Derivative liabilities, not offset	1,757	1,542
Derivative liability, net	(1,335)	0
Derivative asset (liability), gross	9,465	24,682
Derivative asset (liability), amount offset	0	0
Derivative asset (liability), net	9,465	24,682
Derivative asset (liability) not offset	0	0
Derivative asset (liability), net	$ (1,335)	$ 24,682