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Equity Incentive Plans (Summary Of Restricted Share Unit Activity) (Details)
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2018
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Non-employee Trustee Length of Period Subsequent to Grant Date Options Not Exercisable 1 year
Range of settlement date for shares for non-employee trustee from grant date, minimum 1 year
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]  
Number of Shares, Outstanding at December 31, 2017 | shares 620,122
Number of Shares, Granted | shares 295,202
Number of Shares, Vested | shares (244,852)
Number of Shares, Outstanding at September 30, 2018 | shares 655,056
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2017 | $ / shares $ 68.07
Weighted Average Grant Date Fair Value, Granted | $ / shares 56.94
Weighted Average Grant Date Fair Value, Vested | $ / shares 65.33
Weighted Average Grant Date Fair Value, Outstanding at September 30, 2018 | $ / shares $ 64.16
Unamortized share-based compensation expense | $ $ 23.2
Weighted Average Life Remaining, Outstanding at September 30, 2018 (in years) 1 year 1 month 23 days
Restricted Share Units [Member]  
Range of settlement date for shares for non-employee trustee from grant date, minimum 1 year
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]  
Number of Shares, Outstanding at December 31, 2017 | shares 19,030
Number of Shares, Granted | shares 23,571
Number of Shares, Vested | shares (19,030)
Number of Shares, Outstanding at September 30, 2018 | shares 23,571
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2017 | $ / shares $ 70.91
Weighted Average Grant Date Fair Value, Granted | $ / shares 61.25
Weighted Average Grant Date Fair Value, Vested | $ / shares 70.91
Weighted Average Grant Date Fair Value, Outstanding at September 30, 2018 | $ / shares $ 61.25
Unamortized share-based compensation expense | $ $ 1.0
Weighted Average Life Remaining, Outstanding at September 30, 2018 (in years) 7 months 30 days