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Shared-Based Payments (Summary of ESPP and ASPP Shares Issued) (Details) - shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]      
Stock Issued During Period Shares 635,521 720,323 863,426
Employee Stock [Member]      
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]      
Stock Issued During Period Shares 75,093 88,432 100,944
Agent Stock Purchase Plan [Member]      
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]      
Stock Issued During Period Shares 49,794 69,867 82,142