Derivatives and hedging activities - Fair value of derivative financial instruments (Details) - Derivatives designated in a cash flow hedging relationship - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair value of derivative financial instruments
Total fair value, net	$ (4,080)	$ (3,589)
Interest Rate Contract | Other assets
Fair value of derivative financial instruments
Fair value of derivative assets	4	13
Interest Rate Contract | Accrued liabilities
Fair value of derivative financial instruments
Fair value of derivative liabilities	(2,916)	(2,984)
Interest Rate Contract | Other liabilities
Fair value of derivative financial instruments
Fair value of derivative liabilities	$ (1,168)	$ (618)