Equity Incentive Plans (Summary Of Restricted Share Unit Activity) (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 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, 2018 | shares	655,056
Number of Shares, Granted | shares	197,755
Number of Shares, Vested | shares	(337,910)
Number of Shares, Outstanding at March 31, 2019 | shares	507,999
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2018 | $ / shares	$ 64.16
Weighted Average Grant Date Fair Value, Granted | $ / shares	73.84
Weighted Average Grant Date Fair Value, Vested | $ / shares	64.61
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2019 | $ / shares	$ 67.61
Unamortized share-based compensation expense | $	$ 23.5
Weighted Average Life Remaining, Outstanding at March 31, 2019 (in years)	1 year 7 months 7 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, 2018 | shares	23,571
Number of Shares, Granted | shares	1,156
Number of Shares, Vested | shares	0
Number of Shares, Outstanding at March 31, 2019 | shares	24,727
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2018 | $ / shares	$ 61.25
Weighted Average Grant Date Fair Value, Granted | $ / shares	69.11
Weighted Average Grant Date Fair Value, Vested | $ / shares	0.00
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2019 | $ / shares	$ 61.62
Unamortized share-based compensation expense | $	$ 0.3
Weighted Average Life Remaining, Outstanding at March 31, 2019 (in years)	2 months