Share-Based Compensation (Tables)
|3 Months Ended|
Mar. 31, 2018
|Share-based Compensation [Abstract]|
|Fair Value Assumptions for Performance Awards||
The fair values of the senior executives' performance share award grants with a TSR modifier for grants in 2018 and 2017 were $61.59 and $64.82, respectively, which were estimated using the Monte Carlo valuation model, and incorporated the following assumptions:
Schedule of Share-based Payment Award, Equity Instruments Other than Options, Valuation Assumptions [Table Text Block]
No definition available.