Equity Incentive Plans (Summary Of Restricted Share Unit Activity) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Mar. 31, 2012	Jun. 30, 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	350,863
Number of Shares, Granted		148,095
Number of Shares, Vested		(175,965)
Number of Shares, Outstanding at March 31, 2012		322,808
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2011	$ 38.11	$ 38.11
Weighted Average Grant Date Fair Value, Granted		$ 45.20
Weighted Average Grant Date Fair Value, Vested		$ 35.97
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2012		$ 42.52
Weighted Average Life Remaining, Outstanding at March 31, 2012 (in years)		1.38
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)		1 year
Unamortized share-based compensation expense		$ 8,400
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	10,519
Number of Shares, Granted		10,925
Number of Shares, Vested		(10,519)
Number of Shares, Outstanding at March 31, 2012		10,925
Weighted Average Grant Date Fair Value, Outstanding at December 31, 2011	$ 47.77	$ 47.77
Weighted Average Grant Date Fair Value, Granted		$ 44.62
Weighted Average Grant Date Fair Value, Vested		$ 47.77
Weighted Average Grant Date Fair Value, Outstanding at March 31, 2012		$ 44.62
Weighted Average Life Remaining, Outstanding at March 31, 2012 (in years)		0.86
Range of settlement date for shares for non-employee trustee from grant date, minimum (in years)	1 year
Unamortized share-based compensation expense		$ 406