Accumulated other comprehensive income/loss - Reclassifications out of accumulated other comprehensive income/(loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cash flows from derivative financial instruments related to interest payments made for hedged debt instruments	$ 7,867	$ 7,676	$ 15,543	$ 15,186
Net (losses)/earnings	(4,971)	8,308	(22,948)	6,839
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (losses)/earnings	462	703	979	1,412
Reclassification out of Accumulated Other Comprehensive Income | Derivative financial instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cash flows from derivative financial instruments related to interest payments made for hedged debt instruments	$ 462	$ 703	$ 979	$ 1,412