Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified			1 Months Ended	12 Months Ended		1 Months Ended				0 Months Ended	1 Months Ended		0 Months Ended	1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Mar. 31, 2012 RMR	Dec. 31, 2011 RMR	Mar. 29, 2012 RMR	Jan. 31, 2012 Series B	Aug. 31, 2012 Series C	May 31, 2012 Series C	Feb. 29, 2012 Series C	Oct. 15, 2012 Series D	Jul. 31, 2012 Series D	Apr. 30, 2012 Series D	Sep. 14, 2012 Common Shares	Aug. 31, 2012 Common Shares	May 31, 2012 Common Shares	Feb. 29, 2012 Common Shares	May 09, 2012 Common Shares trustee	Oct. 01, 2012 Distribution declared in October 2012 Series C	Oct. 09, 2012 Distribution declared in October 2012 Common Shares
Distributions
Distribution to preferred shareholders (in dollars per share)						$ 0.5546875	$ 0.4375	$ 0.4375	$ 0.4375	$ 0.4453125	$ 0.4453125	$ 0.43046875
Distribution declared per share (in dollars per share)																		$ 0.4375	$ 0.47
Distribution to common shareholders (in dollars per share)														$ 0.45	$ 0.45	$ 0.45
Common Share Issuances
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)													$ 25.02				$ 25.58
Common shares granted to trustees in period													74,017		2,000
Common shares issued for services rendered by RMR			33,132
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01			$ 0.01
Number of Trustees																	5
Incentive fee earned by RMR				$ 741