Derivatives and hedging activities - Effect of derivative financial instruments on the statements of consolidated operations and statement of consolidated comprehensive income (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest rate swaps designated as hedging instruments:
Ending accumulated other comprehensive loss	$ (5,976,000)	$ (6,440,000)
Credit-risk-related contingent features
Fair value of derivatives in a net liability position	(8,879,000)	(10,954,000)
Cash collateral posted with derivative counterparties	0	0
Assets required to settle obligations under derivatives with credit-risk-related contingent features upon breach of provisions, termination value	8,946,000	11,551,000
Interest Rate Swaps
Interest rate swaps designated as hedging instruments:
Beginning accumulated other comprehensive loss	(6,440,000)	(8,745,000)	(11,275,000)
Amount of loss recognized in OCI (effective portion)	(7,032,000)	(7,566,000)	(8,385,000)
Amount of loss reclassified from accumulated OCI into interest income (effective portion)	6,129,000	8,789,000	11,027,000
Deferred tax on OCI movement	1,367,000	1,082,000	(112,000)
Change in fair value of derivatives, net of tax of ($XXX), ($1,082) and $112	464,000	2,305,000	2,530,000
Ending accumulated other comprehensive loss	(5,976,000)	(6,440,000)	(8,745,000)
Amount of loss recognized in interest expense on derivatives (ineffective portion)	218,000	(353,000)	(534,000)
Derivatives not designated as hedging instruments:
Amount of (loss)/gain recognized in interest expense	(54,000)	484,000	(2,217,000)
Amount recorded in other comprehensive income which is expected to be reclassified to interest expense in the next 12 months	$ 3,602,000	$ 3,080,000