Derivatives and hedging activities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Gain/(loss) recorded in other comprehensive income/(loss)	$ (6,488)	$ 1,359	$ 13,666	$ 1,039
Fair value of derivatives in a net liability position	(4,084)		(4,084)		$ (3,602)
Assets required to settle obligations under derivatives with credit-risk-related contingent features upon breach of provisions, termination value	4,085		4,085		3,615
Net Investment Hedging
Derivatives, Fair Value [Line Items]
Fair value of non-derivative hedging instruments			165,626		$ 180,149
Interest rate swaps
Derivatives, Fair Value [Line Items]
Amount recorded in other comprehensive income which is expected to be reclassified to interest expense in the next 12 months	$ 2,854		$ 2,854