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Debt and Credit Agreements (Details) - Line of Credit [Member] - USD ($)
Dec. 31, 2017
Dec. 02, 2015
Dec. 01, 2015
Line of Credit Facility [Line Items]      
Facility Limit $ 400,000,000 $ 600,000,000 $ 500,000,000
Outstanding Balance 41,000,000    
Outstanding Letters of Credit 31,432,000    
Available Credit $ 327,568,000