Derivatives and hedging activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Gain (loss) recorded in other comprehensive income/(loss)	$ 4,975	$ 18,221	$ 24,816
Fair value of derivatives in a net liability position	3,364	4,464
Assets required to settle obligations under derivatives with credit-risk-related contingent features upon breach of provisions, termination value	3,370	4,513
Fair value of non-derivative hedging instruments	153,472	$ 160,138
Interest rate swaps
Derivative [Line Items]
Amount accounted for in other comprehensive income/(loss), which is expected to be reclassified to interest expense in the next 12 months	$ 2,266