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Fair Value of Financial Instruments - Fair Value of Certain Financial Assets and Liabilities and Other Financial Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value $ 937,792 $ 920,539
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value 937,792 920,539
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value $ 942,529 $ 906,156