Equity Incentive Plans (Summary Of Share Option Activity) (Details) (USD $)	3 Months Ended		144 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2012 Y	Mar. 31, 2011	Dec. 31, 2008	Dec. 31, 2011	Mar. 31, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Mar. 31, 2012 Maximum [Member]	Dec. 31, 2011 Maximum [Member]	Mar. 31, 2012 2007 Equity Incentive Plan [Member]	May 09, 2007 2007 Equity Incentive Plan [Member]
Common shares, options to purchase common shares and restricted share units, expected to granted (in shares)										1,950,000
Number of shares available for grant (in shares)									449,032
Maximum term of options granted (in years)	10
Exercisable period for employee options (in years)	4 years		5 years
Exercisable rate for employees options, per year	25.00%		20.00%
Length of period subsequent to grant date options not exercisable for non-employee trustees (in years)	1 year
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Shares, Outstanding at Beginning of Period	1,002,833
Number of Shares, Exercised	(49,597)
Number of Shares, Granted	76,617
Number of Shares, Forfeited	(338)
Number of Shares, Outstanding at End of Period	1,029,515
Option Price Per Share, Outstanding at Beginning of Period					$ 18.18	$ 18.18	$ 65.50	$ 65.50
Option Price Per Share, Exercised					$ 18.18		$ 36.56
Option Price Per Share, Granted					$ 45.20		$ 45.78
Option Price Per Share, Forfeited					$ 18.18		$ 46.69
Option Price Per Share, Outstanding at End of Period					$ 18.18	$ 18.18	$ 65.50	$ 65.50
Average Exercise Price, Outstanding at Beginning of Period	$ 35.97			$ 34.41
Average Exercise Price, Exercised	$ 18.65
Average Exercise Price, Granted	$ 45.24
Average Exercise Price, Forfeited	$ 38.89
Average Exercise Price, Outstanding at End of Period	$ 35.97			$ 34.41
Weighted average fair value of options granted	$ 11.98	$ 9.30
Intrinsic value of stock options exercised	$ 1,300,000	$ 1,700,000
Repurchase of treasury stock (in shares)	30,758
Repurchase of treasury stock, value	1,400,000
Share based compensation expenses recognized in future periods	$ 1,600,000