Derivatives and hedging activities - Notional amounts of outstanding interest rate derivatives (Details) € in Thousands, $ in Thousands	Jun. 30, 2017 USD ($)	Jun. 30, 2017 EUR (€)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)
Interest rate swaps
Derivatives and hedging activities
Derivative, notional amount	$ 209,894	€ 72,563	$ 210,756	€ 72,938
Interest rate caps
Derivatives and hedging activities
Derivative, notional amount	$ 17,200		$ 17,200