Derivative Instruments - Cash flow hedges (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	$ (309)	$ 458	$ 874	$ 780
Cash Flow Hedging
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	221	164	1,636	160
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	113	(117)	182	(517)
Cash Flow Hedging | Interest rate contract | Interest expense [Member]
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	221	164	1,636	160
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 113	$ (117)	$ 182	$ (517)