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Derivative Instruments (Fair Value of Derivative Instruments) (Details) - Designated as Hedging Instrument [Member] - Cash Flow Hedging [Member] - Interest rate contracts [Member] - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Other Noncurrent Assets [Member]    
Derivatives Fair Value [Line Items]    
Asset Derivatives, Fair Value $ 113 $ 1,510
Other Long-term Obligations [Member]    
Derivatives Fair Value [Line Items]    
Liability Derivatives, Fair Value $ 21,333 $ 2,888