Organization (Details) ft² in Millions	12 Months Ended
	Dec. 31, 2015 ft² state property building $ / shares shares	Dec. 31, 2014 $ / shares
Organization
Number of properties owned | property	71
Number of buildings | building	91
Number of states in which owned properties located | state	31
Rentable area of properties (in square feet) | ft²	10.7
Common shares of beneficial interest, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Discontinued operations
Organization
Number of properties owned | property	1
Number of buildings | building	1
SIR
Organization
SIR Common shares of beneficial interest acquired pursuant to stock purchase agreement | shares	24,918,421
Common shares of beneficial interest, par value (in dollars per share) | $ / shares	$ 0.01
Percentage of ownership interest	27.90%