Derivatives and hedging activities (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value of derivative financial instruments
Fair value of assets	$ 10	$ 60
Fair value of liabilities	(9,156)	(10,954)
Total	(9,146)	(10,894)
Derivatives designated in a cash flow hedging relationship
Fair value of derivative financial instruments
Total	(9,156)	(10,954)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Accrued liabilities
Fair value of derivative financial instruments
Fair value of liabilities	(3,942)	(3,443)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Other liabilities
Fair value of derivative financial instruments
Fair value of liabilities	(5,214)	(7,511)
Derivatives not designated as hedges
Fair value of derivative financial instruments
Total	10	60
Derivatives not designated as hedges | Interest Rate Options | Other assets
Fair value of derivative financial instruments
Fair value of assets	$ 10	$ 60