Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		1 Months Ended		3 Months Ended					12 Months Ended	1 Months Ended				3 Months Ended	1 Months Ended	3 Months Ended
	Mar. 31, 2012 sqft	Mar. 31, 2011 sqft	Mar. 31, 2012 SIR	Feb. 29, 2012 SIR	Mar. 31, 2012 SIR trustee	Mar. 31, 2012 Sale SIR sqft property	Mar. 31, 2012 RMR agreement	Mar. 31, 2011 RMR	Mar. 31, 2012 GOV trustee	Dec. 31, 2010 GOV Sale property	Jun. 30, 2011 AIC Y	Mar. 31, 2012 AIC	Jun. 30, 2010 AIC	Mar. 31, 2012 AIC Maximum	Mar. 31, 2012 SNH trustee	Sep. 30, 2011 SNH Sale state property sqft	Mar. 31, 2011 SNH Sale	Jan. 31, 2011 SNH Sale property sqft
Related person transactions
Number of agreements							2
Business management fees					$ 247		$ 10,383	$ 9,228
Property management fees					158		7,924	6,842
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	74,125,000	67,550,000				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																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
Equity method investments, ownership percentage									21.10%			14.30%		20.00%
Consolidation Less than Wholly Owned Subsidiary Parent Ownership Percentage			70.50%		70.50%
Less than wholly owned subsidiary, market value based on quoted market prices			496,760		496,760
Less than Wholly Owned Subsidiary Quoted Market Price Per Share			$ 22.58		$ 22.58
Issuance of shares, net			9,200,000
Common shares consideration received on transfer of properties			22,000,000		22,000,000
Number of commons shares in initial public offering pursuant to underwriters over allotment option			1,200,000		1,200,000
Proceeds from Issuance Initial Public Offering			180,954		180,954
Repayment of promissory note			400,000		400,000
Issuance of promissory note				400,000
Maximum borrowing capacity			500,000		500,000
Coverage amount for property insurance													500,000
Period for which property insurance program was extended (in years)											1
Premium for property insurance											$ 5,540
Number of states where properties are located																8