Equity Incentive Plans (Summary Of Restricted Share Unit Activity) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Y
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Number of Shares, Outstanding at December 31, 2011	350,863
Number of Shares, Granted	148,095
Number of Shares, Vested	(170,881)
Number of Shares, Outstanding at March 31, 2012	327,892
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2011	$ 38.11
Weighted Average Grant Date Fair Value, Granted	$ 45.20
Weighted Average Grant Date Fair Value, Vested	$ 35.84
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2012	$ 42.49
Weighted Average Life Remaining, Outstanding at March 31, 2012 (in years)	1.63
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)	1 year
Unamortized share-based compensation expense	$ 9,600
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, 2011	10,519
Number of Shares, Granted	0
Number of Shares, Vested	0
Number of Shares, Outstanding at March 31, 2012	10,519
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2011	$ 47.77
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, 2012	$ 47.77
Weighted Average Life Remaining, Outstanding at March 31, 2012 (in years)	0.10
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)	1 year
Unamortized share-based compensation expense	$ 42