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 |