Equity Incentive Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Equity Incentive Plan
Maximum number of common shares of beneficial interest to be issued (in shares)	5,000,000
Summary of non-vested shares/units
Granted (in shares)	4,347	[1]	5,357	[1]	8,530	[1]
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 29.31	[1]	$ 23.20	[1]	$ 18.75	[1]
Other Disclosures
Share-based compensation expense	$ 127,000		$ 124,000		$ 160,000
Restricted share awards
Summary of non-vested shares/units
Unvested at the beginning of the period (in shares)	932,800		1,013,673		970,288
Granted (in shares)	348,234	[1]	383,187	[1]	445,176	[1]
Vested (in shares)	(537,988)	[1]	(458,808)	[1]	(374,913)	[1]
Forfeited (in shares)	(11,587)		(5,252)		(26,878)
Unvested at the end of the period (in shares)	731,459		932,800		1,013,673
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 18.99		$ 17.80		$ 17.93
Granted (in dollars per share)	$ 24.46	[1]	$ 21.25	[1]	$ 17.23	[1]
Vested (in dollars per share)	$ 19.44	[1]	$ 18.24	[1]	$ 17.46	[1]
Forfeited (in dollars per share)	$ 22.89		$ 20.07		$ 17.80
Unvested at the end of the period (in dollars per share)	$ 21.21		$ 18.99		$ 17.80
Other Disclosures
Share-based compensation expense	10,600,000		8,600,000		6,700,000
Total unrecognized compensation costs	14,200,000
Weighted-average period of recognition of unrecognized share-based compensation expense	2 years 2 months
Total fair value of shares vested	15,500,000		10,500,000		6,500,000
Performance Units
Summary of non-vested shares/units
Unvested at the end of the period (in shares)	1,000,000
Weighted Average Grant Date Fair Value
Unvested at the end of the period (in dollars per share)	$ 15.36
Other Disclosures
Share-based compensation expense	4,500,000		4,500,000		1,900,000
Total unrecognized compensation costs	$ 4,500,000
Weighted-average period of recognition of unrecognized share-based compensation expense	1 year 1 month 6 days
Vesting period	4 years
Measurement period	3 years
Time-based vesting period	1 year
Common shares available for future grant (in share)	2,741,480

[1] For the years ended December 31, 2014, 2013 and 2012, respectively, includes 4,347, 5,357 and 8,530 unrestricted shares issued in lieu of cash compensation to non-employee trustees at a weighted average grant date fair value of $29.31, $23.20 and $18.75, respectively.