Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 sqft property	Sep. 30, 2012 sqft	Sep. 30, 2013 sqft property	Sep. 30, 2012 sqft	Mar. 31, 2013 property	Dec. 31, 2012 property
Segment information
Number of reportable segments			2
Number of geographical regions comprised mainly by properties owned by SIR			1
Number of remaining geographical regions			2
Square feet	38,895,000	61,733,000	38,895,000	61,733,000
Rental income	$ 201,741	$ 226,802	$ 699,207	$ 662,587
NOI	107,109	134,797	411,557	397,360
Investment in properties, net of accumulated depreciation	4,757,038		4,757,038			6,821,803
CBD Properties
Segment information
Number of Real Estate Properties	38		38
Number of buildings	51		51
Number of Real Estate Properties Reclassified to Discontinued Operations	3		3			1
Number of buildings reclassified to discontinued operations	4		4			1
Number of Real Estate Properties Reclassified from One Geographical Segment to Another Geographical Segment					3
Number of buildings reclassified from one geographical segment to another geographical segment					3
Square feet	20,930,000	20,166,000	20,930,000	20,166,000
Rental income	136,228	130,193	413,550	383,245
NOI	68,663	70,208	220,182	210,250
Investment in properties, net of accumulated depreciation	2,999,964		2,999,964
CBD Properties | Australia
Segment information
Investment in properties, net of accumulated depreciation	149,075		149,075
Suburban Properties
Segment information
Number of Real Estate Properties	90		90
Number of buildings	184		184
Number of Real Estate Properties Reclassified to Discontinued Operations	42		42			39
Number of buildings reclassified to discontinued operations	106		106			93
Number of Real Estate Properties Reclassified from One Geographical Segment to Another Geographical Segment					3
Number of buildings reclassified from one geographical segment to another geographical segment					3
Square feet	17,965,000	41,567,000	17,965,000	41,567,000
Rental income	65,513	96,609	285,657	279,342
NOI	38,446	64,589	191,375	187,110
Investment in properties, net of accumulated depreciation	1,757,074		1,757,074
Suburban Properties | Australia
Segment information
Investment in properties, net of accumulated depreciation	$ 92,956		$ 92,956
Suburban Properties | Minimum
Segment information
Percentage of real estate properties that are office properties			90.00%