Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details)

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Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - Recurring Basis - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Assets    
Fair value of derivatives $ 406 $ 103
Liabilities    
Fair value of derivatives (29,028) (18,296)
Total net liabilities (28,622) (18,193)
Level 1    
Assets    
Fair value of derivatives 0 0
Liabilities    
Fair value of derivatives 0 0
Total net liabilities 0 0
Level 2    
Assets    
Fair value of derivatives 406 103
Liabilities    
Fair value of derivatives (29,028) (18,296)
Total net liabilities (28,622) (18,193)
Level 3    
Assets    
Fair value of derivatives 0 0
Liabilities    
Fair value of derivatives 0 0
Total net liabilities $ 0 $ 0