Derivatives and hedging activities - Notional amounts of outstanding interest rate derivatives (Details) € in Thousands, $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)
Interest rate swaps
Derivative
Derivative, notional amount	$ 243,000	€ 89,500	$ 210,756	€ 72,938
Interest rate caps
Derivative
Derivative, notional amount	$ 17,200		$ 17,200