Derivatives and hedging activities (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Interest rate swaps designated as hedging instruments:
Deferred tax on OCI movement	$ (1,494,000)	$ 50,000	$ (643,000)	$ 190,000
Interest rate swaps not designated as hedging instruments:
Fair value of derivatives in a net liability position	11,093,000		11,093,000
Cash collateral posted with derivative counterparties					1,558,000
Assets required to settle obligations under derivatives with credit-risk-related contingent features upon breach of provisions, termination value	11,730,000		11,730,000
Interest Rate Swaps
Interest rate swaps designated as hedging instruments:
Amount of gain/(loss) recognized in OCI (effective portion)	(6,801,000)	(2,405,000)	(6,035,000)	(9,154,000)
Amount of loss reclassified from accumulated OCI into income (effective portion)	(3,965,000)	(2,513,000)	(7,928,000)	(7,714,000)
Deferred tax on OCI movement	(1,494,000)	50,000	(643,000)	190,000
Amount of loss recognized in interest expense on derivatives (ineffective portion)	(209,000)	(147,000)	(375,000)	(447,000)
Interest rate swaps not designated as hedging instruments:
Amount of gain/(loss) recognized in interest expense	479,000	(318,000)	524,000	(1,136,000)
Amount recorded in other comprehensive income which is expected to be reclassified to interest expense in the next 12 months	$ 3,178,000		$ 3,178,000