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Debt and Credit Agreements (Details) - Line of Credit [Member] - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Dec. 02, 2015
Dec. 01, 2015
Line of Credit Facility [Line Items]        
Facility Limit $ 400,000,000 $ 350,000,000 $ 600,000,000 $ 500,000,000
Outstanding Balance 98,000,000      
Outstanding Letters of Credit 43,549,000      
Available Credit $ 258,451,000