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Stock-Based Compensation (Performance Shares) (Details) - Performance share awards [Member]
$ / shares in Units, $ in Millions
12 Months Ended 36 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Target performance share awards granted   41,439
Granted, Weighted Average Grant Date Value per RSU | $ / shares $ 81.71  
Number of shares that will be issued based on final 2018 performance levels 0  
Unamortized expense, based on current performance levels (in millions) | $ $ 0.0 $ 0.0