Derivative Instruments - Cash flow hedges (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	$ 1,616	$ 434
Cash Flow Hedging
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	1,031	335
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	24	226
Cash Flow Hedging | Interest rate contracts | Interest expense Member
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	1,031	335
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 24	$ 226