Accumulated other comprehensive income/loss - Reclassifications out of accumulated other comprehensive income/(loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
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	$ 8,521	$ 8,344	$ 18,056	$ 15,597
Total reclassifications for the period	5,996	17,337	(14,648)	(29,669)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	595	882	1,205	1,783
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	$ 595	$ 882	$ 1,205	$ 1,783