Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended		1 Months Ended		6 Months Ended	1 Months Ended	3 Months Ended		6 Months Ended			12 Months Ended	6 Months Ended		1 Months Ended	3 Months Ended
	Jun. 30, 2012 sqft	Jun. 30, 2012 sqft	Jun. 30, 2011 sqft	Mar. 31, 2012 SIR	Feb. 29, 2012 SIR	Jun. 30, 2012 SIR trustee	Feb. 29, 2012 Sale SIR property sqft	Jun. 30, 2012 RMR agreement	Jun. 30, 2011 RMR	Jun. 30, 2012 RMR trustee agreement	Jun. 30, 2011 RMR	Jun. 30, 2012 GOV trustee	Dec. 31, 2010 GOV Sale property	Jun. 30, 2012 AIC trustee	Jun. 30, 2012 SNH trustee	Sep. 30, 2011 SNH Sale property state sqft	Mar. 31, 2011 SNH Sale	Jan. 31, 2011 SNH Sale property sqft
Related person transactions
Number of agreements								2		2
Business management fees								$ 10,647	$ 9,500	$ 21,030	$ 18,728
Property management fees								7,966	6,811	15,890	13,653
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	74,753,000	74,753,000	69,044,000				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	350	350														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		4	2
Equity method investments, ownership percentage												21.10%		12.50%
Less than wholly owned subsidiary, parent ownership percentage				70.50%		70.50%
Less than wholly owned subsidiary, market value based on quoted market prices						522,720
Less than wholly owned subsidiary, quoted market price of common shares (in dollars per share)						$ 23.76
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
Number of states where properties are located																8