Annual report pursuant to Section 13 and 15(d)

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

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Fair Value Measurement - Fair Value of Assets and Liabilities Measured on a Recurring Basis (Details) - Recurring - Fair Value, Inputs, Level 2 - Interest Rate Swap - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Assets:    
Interest rate swap instruments $ 233  
Liabilities:    
Interest rate swap instruments $ 3,529 $ 3,843