Fair value measurements (Derivatives offset in the consolidated balance sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative assets
Offset in the consolidated balance sheets, Gross assets (liabilities)	$ 29,622	$ 1,332
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	(3,398)	(1,332)
Offset in the consolidated balance sheets, Net assets (liabilities)	26,224	0
Gross amounts not offset in the consolidated balance sheets, Derivatives	(1,542)	0
Gross amounts not offset in the consolidated balance sheets, Amounts outstanding under credit facilities	0	0
Gross amounts not offset in the consolidated balance sheets, Net amount	24,682	0
Derivative liabilities
Offset in the consolidated balance sheets, Gross assets (liabilities)	(4,940)	(14,458)
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	3,398	1,332
Offset in the consolidated balance sheets, Net assets (liabilities)	(1,542)	(13,126)
Gross amounts not offset in the consolidated balance sheets, Derivatives	1,542	0
Gross amounts not offset in the consolidated balance sheets, Amounts outstanding under credit facilities	0	0
Gross amounts not offset in the consolidated balance sheets, Net amount	0	(13,126)
Offset in the consolidated balance sheets, Gross assets (liabilities)	24,682	(13,126)
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	0	0
Offset in the consolidated balance sheets, Net assets (liabilities)	24,682	(13,126)
Gross amounts not offset in the consolidated balance sheets, Derivatives	0	0
Gross amounts not offset in the consolidated balance sheets, Net amount	$ 24,682	$ (13,126)