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Share-Based Employee Compensation (Employee Stock Purchase Plan) (Details) - ESPP - USD ($)
9 Months Ended
Sep. 13, 2012
Dec. 31, 2017
Sep. 12, 2012
Jun. 18, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for issuance under the amended plan (in shares) 14,000,000.0   9,000,000.0 9,000,000.0
Limit of fair market value any one employee can purchase per year       $ 25,000
Additional shares authorized for future issuance (in shares) 5,000,000.0      
Number of shares issued (in shares)   293,000    
Average issuance price (in dollars per share)   $ 21.23    
Number of shares available   2,971,000