Related Person Transactions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended			0 Months Ended		6 Months Ended						3 Months Ended		6 Months Ended		0 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013 trustee item sqft	Dec. 31, 2012	Jun. 30, 2012 sqft	Mar. 12, 2012 SIR	Feb. 16, 2012 SIR property	Jun. 30, 2013 SIR item	Jul. 02, 2013 SIR	Jun. 30, 2013 Sale sqft	May 31, 2013 Sale sqft	Jan. 31, 2013 Sale sqft	Feb. 16, 2012 Sale SIR sqft	Jun. 30, 2013 RMR item	Jun. 30, 2012 RMR	Jun. 30, 2013 RMR item	Jun. 30, 2012 RMR	Mar. 15, 2013 GOV	Jun. 30, 2013 GOV	Jun. 30, 2013 AIC company	Jun. 30, 2013 RMR Australia	Jun. 30, 2013 RMR Australia
Related person transactions
Number of employees	0
Number of agreements												2		2
Business management fees												$ 12,051	$ 10,647	$ 23,956	$ 21,030				$ 445
Property management fees												8,608	7,966	16,984	15,890					765
Business management fees, excluding fees recognized by SIR												9,792	9,395	19,530	19,531
Property management fees, construction supervision fees excluding fees recognized by SIR												7,297	7,192	14,415	14,958
Ownership interest in subsidiary before spin-off (as a percent)						100.00%											100.00%
Equity investments, common shares owned																9,950,000
Net proceeds received on sale of common shares																239,576
Amount of reimbursements payable to related party	12,954	12,958				622											303
Less than wholly owned subsidiary, common shares owned						22,000,000
Less than wholly owned subsidiary, parent ownership interest percentage						56.00%	44.20%
Number of common managing trustees	2
Issuance of shares, net				9,200,000
Proceeds from Issuance Initial Public Offering				180,814
Repayment of promissory note					400,000
Common shares consideration received on transfer of properties					22,000,000
Number of properties sold or transferred or agreed to be sold or transferred					251
Property square feet	72,717,000		67,870,000					2,265,228	57,250	1,060,026	21,400,000
Number of other companies which are shareholders of related party																		4
Equity method investments, ownership percentage																		12.50%
Amount invested in equity investee																		10,544
Coverage amount for property insurance																		500,000
Premium for property insurance																		$ 5,984
Period for which property insurance program was extended																		1 year
Number of independent trustees						1