Derivatives and hedging activities - Fair value of derivative financial instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives designated in a cash flow hedging relationship
Fair value of derivative financial instruments
Total	$ (8,879)	$ (10,954)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Other assets
Fair value of derivative financial instruments
Fair value of assets	0	0
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Accrued liabilities
Fair value of derivative financial instruments
Fair value of liabilities	(3,858)	(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	(5,021)	(7,511)
Derivatives not designated as hedges
Fair value of derivative financial instruments
Total	6	60
Derivatives not designated as hedges | Interest Rate Swaps | Accrued liabilities
Fair value of derivative financial instruments
Fair value of liabilities	0	0
Derivatives not designated as hedges | Interest Rate Swaps | Other liabilities
Fair value of derivative financial instruments
Fair value of liabilities	0	0
Derivatives not designated as hedges | Interest Rate Options | Other assets
Fair value of derivative financial instruments
Fair value of assets	$ 6	$ 60