Derivative instruments (Amount and balance sheet location) (Details) $ in Thousands	Dec. 31, 2016 USD ($) derivatives	Dec. 31, 2015 USD ($)
Interest rate swap contracts
Summary of amount of derivatives designated as hedging instruments
Derivative, Number of Instruments Held | derivatives	2
Not designated as hedging instruments | Foreign currency forward contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount	$ 181,375	$ 255,840
Not designated as hedging instruments | Foreign currency forward contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	2,351	2,601
Not designated as hedging instruments | Foreign currency forward contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	3,421	3,211
Designated as hedging instruments | Interest rate swap contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount	552,263	552,263
Designated as hedging instruments | Interest rate swap contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	20	21
Designated as hedging instruments | Interest rate swap contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	$ 1,479	$ 1,942