Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended			3 Months Ended				1 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 RMR Agreement entity	Mar. 31, 2011 RMR	Mar. 31, 2012 CWH	Dec. 31, 2010 CWH Acquisition property	Jun. 30, 2010 AIC	Mar. 31, 2012 AIC	Mar. 31, 2011 AIC	Dec. 31, 2011 AIC	Mar. 31, 2012 AIC Maximum
Related Party Transaction
Number of agreements to avail management and administrative services				2
Percentage of interest in subsidiaries						100.00%
Percentage of outstanding shares owned						21.10%
Expenses incurred pursuant to our business management agreement				$ 2,231	$ 1,674
Common shares of beneficial interest issued				39,141
Property management and construction supervision fees incurred				1,590	1,324
Number of other companies owning interest in equity method investment				4
Number of properties acquired or agreed to be acquired							15
Common shares owned	47,090,791		47,051,650			9,950,000
Investment at carrying value									5,454		5,409
Recognized income (loss) related to investment in AIC	45	37							45	37
Coverage of purchased property insurance								500,000
Premium for property insurance									$ 1,286
Percentage of interest						21.10%			14.30%			20.00%