Derivatives and hedging activities - Notional amounts of outstanding interest rate derivatives (Details) € in Thousands, $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 EUR (€)
Interest rate swaps
Derivative [Line Items]
Derivative, notional amount	$ 210,756	€ 72,938	$ 212,481	€ 73,688
Interest rate caps
Derivative [Line Items]
Derivative, notional amount	$ 17,200		$ 17,200