Derivatives and hedging activities - Narratives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative
Gain (loss) recorded in other comprehensive income/(loss)	$ (2,745)	$ 4,975	$ 18,221
Fair value of derivatives in a net asset position	918
Fair value of derivatives in a net liability position		3,364
Assets required to settle obligations under derivatives with credit-risk-related contingent features upon breach of provisions, termination value	(942)	(3,370)
Fair value of non-derivative hedging instruments	213,350	$ 153,472
Interest rate swaps
Derivative
Loss on contract termination	2,145
Amount accounted for in other comprehensive income/(loss), which is expected to be reclassified to interest expense in the next 12 months	$ 2,102