Equity Incentive Plans (Summary Of Restricted Share Unit Activity) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Number of Shares, Outstanding at December 31, 2012	322,808
Number of Shares, Granted	198,833
Number of Shares, Vested	(145,570)
Number of Shares, Outstanding at March 31, 2013	371,864
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2012	$ 42.52
Weighted Average Grant Date Fair Value, Granted	$ 47.15
Weighted Average Grant Date Fair Value, Vested	$ 39.88
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2013	$ 46.00
Weighted Average Life Remaining, Outstanding at March 31, 2013 (in years)	1 year 8 months 25 days
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)		1 year
Unamortized share-based compensation expense	$ 11,900
Restricted Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Number of Shares, Outstanding at December 31, 2012	10,925
Number of Shares, Granted	0
Number of Shares, Vested	0
Number of Shares, Outstanding at March 31, 2013	10,925
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2012	$ 44.62
Weighted Average Grant Date Fair Value, Granted	$ 0.00
Weighted Average Grant Date Fair Value, Vested	$ 0.00
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2013	$ 44.62
Weighted Average Life Remaining, Outstanding at March 31, 2013 (in years)	1 month 5 days
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)	1 year
Unamortized share-based compensation expense	$ 41