Related Person Transactions (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended				1 Months Ended				12 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2010 sqft	Dec. 31, 2012 sqft	Dec. 31, 2011 sqft	Mar. 31, 2012 SIR	Jan. 31, 2013 SIR sqft	Dec. 31, 2012 SIR sqft	Feb. 29, 2012 SIR	Feb. 29, 2012 SIR Suburban office and industrial properties property	Jan. 31, 2013 Sale property sqft	Feb. 29, 2012 Sale SIR property sqft	Feb. 28, 2013 Agreement to sell property sqft	Dec. 31, 2012 GOV	Jun. 30, 2010 GOV Sale	Jun. 30, 2010 GOV Agreement to sell property sqft	Dec. 31, 2012 SNH	Sep. 30, 2011 SNH Sale property sqft	Jan. 31, 2011 SNH Sale property sqft	Dec. 31, 2008 SNH Agreement to sell property
Related person transactions
Percentage of ownership interest of entity in related party												10.00%			10.00%
Number of maximum additional buildings, rights of first refusal to purchase																		45
Number of properties sold or agreed to be sold								23	18		2			15		13	27	47
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs									$ 10,250		$ 5,055		$ 231,000			$ 167,000	$ 470,000	$ 562,000
Gain (loss) on sale of properties	34,336												34,336			7,846	168,272
Property square feet	58,736,000	71,901,000	65,311,000		553,799	6,819,000			1,060,026	21,400,000	675,250			1,900,000		1,310,000	2,803,000
Deferred gain on sale of properties													14,588
Number of properties sold or transferred or agreed to be sold or transferred										251
Common shares consideration received on transfer of properties							22,000,000
Non-interest bearing demand promissory note							400,000
Proceeds from Issuance Initial Public Offering				$ 180,814