Derivative Instruments - Cash flow hedges (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	$ (433)	$ (112)	$ 1,183	$ 323
Cash Flow Hedging
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	384	(338)	1,415	(3)
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	(94)	174	(69)	400
Cash Flow Hedging | Interest rate contracts | Interest expense Member
Effects of derivative instruments
Amount of Gain Recognized in OCI on Derivatives (Effective Portion)	384	(338)	1,415	(3)
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (94)	$ 174	$ (69)	$ 400