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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
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	$ 6,480	$ 8,521	$ 14,095	$ 18,056
Total reclassifications for the period	15,334	5,996	5,880	(14,648)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	738	595	1,467	1,205
Derivative financial instruments | Reclassification out of Accumulated Other Comprehensive Income
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	$ 738	$ 595	$ 1,467	$ 1,205