Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended							12 Months Ended
	Dec. 31, 2011 Y	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 RMR D agreement employee	Dec. 31, 2010 RMR	Dec. 31, 2009 RMR	Nov. 01, 2011 RMR	Dec. 31, 2011 RMR Restricted shares Y	Dec. 31, 2010 RMR Restricted shares	Dec. 31, 2009 RMR Restricted shares	Dec. 31, 2011 RMR Maximum D	Dec. 31, 2011 RMR Office sqft	Dec. 31, 2010 RMR Office	Dec. 31, 2009 RMR Office	Dec. 31, 2009 GOV
Related person transactions
Number of agreements												2
Number of employees												740
Annual compensation for real estate located in United States, Puerto Rico or Canada ( as a percent)															0.70%
First limit of investments in United States, Puerto Rico or Canada															$ 250,000
Annual compensation for real estate located in United States, Puerto Rico or Canada for investments exceeding $250,000 ( as a percent)															0.50%
Annual compensation for real estate located outside United States, Puerto Rico and Canada (as a percent)															1.00%
Incentive fee percentage															15.00%
Incentive fees (in dollars per share)																			$ 0.04
Management fees as a percentage of gross rents												3.00%
Business and property management fees												69,518	62,232	62,563
Business and property management fees allocated																							2,400
Construction supervision fees as a percentage of construction costs												5.00%
Payments to MacarthurCook Fund Management Limited												1,856	185
Pro rata share of providing internal audit function												240	213	220
Period by which business management agreement and property management agreement get automatically renewed (in years)	1								1
Notice required for termination of agreement (in days)												5							60
Approximate office space leased by RMR (in square feet)																				24,000
Approximate office space leased by RMR for regional offices, number																				8
Rental income earned	241,552	241,785	217,938	210,673	200,645	196,272	192,930	192,616	911,948	782,463	764,461									566	498	531
Shares granted																73,050	42,375	38,000
Aggregate value of shares granted																$ 1,458	$ 1,157	$ 1,143
Portion of awards vested (as a percent)																20.00%
Portion of awards vested on each of the next four anniversaries of the grant date (as a percent)																20.00%
Period during which awards vest from date of initial grant ( in years)																4