Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended		1 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended				12 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended
	Sep. 30, 2012 sqft	Sep. 30, 2012 sqft	Sep. 30, 2011 sqft	Mar. 31, 2012 SIR	Feb. 29, 2012 SIR	Sep. 30, 2012 SIR trustee	Feb. 16, 2012 SIR	Feb. 29, 2012 Sale SIR property sqft	Sep. 30, 2012 RMR agreement	Sep. 30, 2011 RMR	Sep. 30, 2012 RMR trustee agreement	Sep. 30, 2011 RMR	Sep. 30, 2012 GOV trustee	Nov. 09, 2012 GOV	Dec. 31, 2010 GOV Sale property	Sep. 30, 2012 AIC	Sep. 30, 2012 SNH trustee	Sep. 30, 2011 SNH Sale property sqft	Mar. 31, 2011 SNH Sale	Jan. 31, 2011 SNH Sale property sqft
Related person transactions
Number of agreements									2		2
Business management fees									$ 11,057	$ 10,178	$ 32,087	$ 28,906
Property management fees									8,593	7,725	24,483	21,378
Ownership interest in subsidiary before spin-off (as a percent)						100.00%							100.00%				100.00%
Number of properties sold or agreed to be sold															15			13		27
Number of properties sold or transferred or agreed to be sold or transferred								251
Property square feet	76,832,000	76,832,000	72,287,000					21,400,000										1,310,000		2,803,000
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs																		167,000		470,000
Gain (loss) on sale of properties	1,689	2,039																7,846	34,666	168,272
Equity investments, common shares owned													9,950,000				250,000
Less than wholly owned subsidiary, common shares owned				22,000,000		22,000,000
Number of common managing trustees						2					2		2				2
Equity method investments, ownership percentage														18.20%		12.50%
Less than wholly owned subsidiary, parent ownership percentage						70.40%
Less than wholly owned subsidiary, market value based on quoted market prices						541,640
Less than wholly owned subsidiary, quoted market price of common shares (in dollars per share)						$ 24.62
Issuance of shares, net				9,200,000
Common shares consideration received on transfer of properties							22,000,000
Number of commons shares in initial public offering pursuant to underwriters over allotment option				1,200,000
Proceeds from Issuance Initial Public Offering				180,814
Repayment of promissory note				400,000
Issuance of promissory note					400,000
Maximum borrowing capacity						500,000
Coverage amount for property insurance																500,000
Period for which property insurance program was extended																1 year
Premium for property insurance																$ 6,560