Equity Incentive Plan (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
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)	1,057
Weighted Average Grant Date Fair Value
Granted (in dollars per share)			$ 31.31
Other Disclosures
Share-based compensation expense	$ 33,000		$ 28,000
Restricted share awards
Summary of non-vested shares/units
Unvested at the beginning of the period (in shares)	731,459
Granted (in shares)	254,299	[1]
Vested (in shares)	(139,213)	[1]
Forfeited (in shares)	(229)
Unvested at the end of the period (in shares)	846,316
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 21.21
Granted (in dollars per share)	$ 32.49	[1]
Vested (in dollars per share)	$ 19.69	[1]
Forfeited (in dollars per share)	$ 26.64
Unvested at the end of the period (in dollars per share)	$ 24.84
Other Disclosures
Share-based compensation expense	2,900,000		2,500,000
Total unrecognized compensation costs	19,500,000
Weighted-average period of recognition of unrecognized share-based compensation expense	2 years 10 months 24 days
Total fair value of shares vested	4,600,000
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	1,100,000		1,100,000
Total unrecognized compensation costs	$ 3,400,000
Weighted-average period of recognition of unrecognized share-based compensation expense	9 months 18 days
Vesting period	4 years
Measurement period	3 years
Time-based vesting period	1 year
Common shares available for future grant (in share)	2,487,410

[1] Includes 1,057 unrestricted shares issued in lieu of cash compensation to non-employee trustees at a weighted-average grant date fair value of $31.31.