Fair value measurements (Derivatives Offset in the Consolidated Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative assets
Offset in the consolidated balance sheets, Gross assets (liabilities)	$ 6,811	$ 29,622
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	(5,864)	(3,398)
Offset in the consolidated balance sheets, Net assets (liabilities)	947	26,224
Gross amounts not offset in the consolidated balance sheets, Derivatives	0	(1,542)
Gross amounts not offset in the consolidated balance sheets, Amounts outstanding under credit facilities	(947)	0
Gross amounts not offset in the consolidated balance sheets, Net amount	0	24,682
Derivative liabilities
Offset in the consolidated balance sheets, Gross assets (liabilities)	(22,896)	(4,940)
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	5,864	3,398
Offset in the consolidated balance sheets, Net assets (liabilities)	(17,032)	(1,542)
Gross amounts not offset in the consolidated balance sheets, Derivatives	0	1,542
Gross amounts not offset in the consolidated balance sheets, Amounts outstanding under credit facilities	947	0
Gross amounts not offset in the consolidated balance sheets, Net amount	(16,085)	0
Offset in the consolidated balance sheets, Gross assets (liabilities)	(16,085)	24,682
Offset in the consolidated balance sheets, Offsetting assets (liabilities)	0	0
Offset in the consolidated balance sheets, Net assets (liabilities)	(16,085)	24,682
Gross amounts not offset in the consolidated balance sheets, Derivatives	0	0
Gross amounts not offset in the consolidated balance sheets, Net amount	$ (16,085)	$ 24,682