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Credit Agreement (Details) - USD ($)
$ in Millions
6 Months Ended
Feb. 08, 2011
Jun. 30, 2018
Line of Credit Facility [Line Items]    
Available revolving credit   484544
Bank Of America February Two Thousand Eleven New Credit Agreement    
Line of Credit Facility [Line Items]    
Revolving line of credit 500000  
Letters of credit outstanding   $ 15.5