Fair value measurements (Derivatives Offset in Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 53,121	$ 6,811
Derivative assets, amount offset	0	(5,864)
Derivative assets, net	53,121	947
Derivative assets, not offset	0	0
Credit facility balance available to offset net derivative assets	(44,411)	(947)
Derivative asset, net	8,710	0
Derivative liabilities, gross	0	(22,896)
Derivative liabilities, amounts offset	0	5,864
Derivative liabilities, net	0	(17,032)
Derivative liabilities, not offset	0	0
Derivative liabilities, outstanding under credit facility		947
Derivative liability, net	0	(16,085)
Derivative asset (liability), gross	53,121	(16,085)
Derivative asset (liability), amount offset	0	0
Derivative asset (liability), net	53,121	(16,085)
Derivative asset (liability) not offset	0	0
Derivative asset (liability). outstanding under credit facility	(44,411)	0
Derivative asset (liability), net	$ 8,710	$ (16,085)