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Fair Value of Financial Instruments - Fair Value of Certain Financial Assets and Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value $ 931,637 $ 937,792
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value 931,637 937,792
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value $ 922,418 $ 942,529