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Financial Instruments - Derivatives and Hedging (Fair Value of Interest Rates) (Details) - Interest Rate Swaps - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Gross derivative assets $ 20,171 $ 21,605
Gross derivative liabilities 0 0
Net derivative assets $ 20,171 $ 21,605