Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurement- Fair Value of Liabilities Measured on a Recurring Basis (Details)

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Fair Value Measurement- Fair Value of Liabilities Measured on a Recurring Basis (Details) - Recurring - Level 2 - Interest rate swap instruments - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Assets:    
Interest rate swap instruments $ 1,041 $ 233
Liabilities:    
Interest rate swap instruments $ 2,440 $ 3,529