Derivatives and hedging activities (Details) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 Interest Rate Swaps USD ($)	Dec. 31, 2012 Interest Rate Swaps AUD	Dec. 31, 2012 Interest Rate Swaps EUR (€)	Dec. 31, 2011 Interest Rate Swaps USD ($)	Dec. 31, 2011 Interest Rate Swaps AUD	Dec. 31, 2011 Interest Rate Swaps EUR (€)	Dec. 31, 2012 Interest Rate Options Derivatives not designated as hedges USD ($)	Dec. 31, 2012 Interest Rate Options Derivatives not designated as hedges EUR (€)	Dec. 31, 2011 Interest Rate Options Derivatives not designated as hedges USD ($)	Dec. 31, 2011 Interest Rate Options Derivatives not designated as hedges EUR (€)
Derivatives and hedging activities
Interest rate derivatives				$ 104,259	0	€ 142,094	$ 117,765	10,800	€ 148,332
Fair value of derivatives not designated as hedges										6		60
Notional amounts of derivatives not designated as hedges										53,760	76,469	54,880	43,594
Fair value of non-derivative hedging instruments	44,166	45,919
Amounts recorded in other comprehensive income/(loss)	$ (806)	$ 2,748	$ 4,398