Accumulated other comprehensive loss - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cash flows from derivative financial instruments related to interest payments made for the hedged debt instrument	$ 29,155	$ 32,101	$ 36,767
Total reclassifications for the period	36,433	15,854	$ (1,735)
Reclassification out of AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	2,809	2,949
Interest expense | Derivative financial instruments | Reclassification out of AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cash flows from derivative financial instruments related to interest payments made for the hedged debt instrument	$ 2,809	$ 2,949