Derivative Instruments and Hedging Activities - Effect of Interest Rate Swaps on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
5 year interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate swaps reclassified to earnings	$ 37	$ 94	$ 93	$ 202
10 year interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate swaps reclassified to earnings	(12)	46	(4)	105
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate swaps reclassified to earnings	$ 25	$ 140	$ 89	$ 307