Derivatives and hedging activities - Fair value of derivative financial instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivatives designated in a cash flow hedging relationship
Fair value of derivative financial instruments
Total net derivative liabilities	$ (2,903)	$ (4,890)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Accrued liabilities
Fair value of derivative financial instruments
Fair value of derivative liabilities	(2,381)	(3,012)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Other liabilities
Fair value of derivative financial instruments
Total net derivative liabilities		(1,878)
Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Other assets
Fair value of derivative financial instruments
Total net derivative liabilities	(522)
Derivatives not designated as hedges
Fair value of derivative financial instruments
Total net derivative liabilities	0	2
Derivatives not designated as hedges | Interest Rate Options | Other assets
Fair value of derivative financial instruments
Fair value of derivative assets	$ 0	$ 2