Property Dispositions Reported as Discontinued Operations Property Dispositions Reported as Discontinued Operations (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2011 NumberofBuildings Share_Based_Incentive_Compensation_Plans		Sep. 30, 2010		Sep. 30, 2011 NumberofTradingDays Years RSF NumberofBuildings Share_Based_Incentive_Compensation_Plans		Sep. 30, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings	9				9
Rental income	$ 89,306,000		$ 72,135,000		$ 252,102,000		$ 196,883,000
Tenant reimbursements	7,683,000		6,156,000		21,469,000		18,261,000
Property expenses	19,361,000		15,802,000		54,548,000		42,255,000
Real estate taxes	8,360,000		7,582,000		24,878,000		20,035,000
Depreciation and amortization	36,152,000		29,951,000		97,513,000		74,405,000
Income from discontinued operations before net gain on dispositions of discontinued operations	308,000		350,000		1,053,000		1,011,000
Net gain on dispositions of discontinued operations	12,555,000		0		12,555,000		0
Total income from discontinued operations	12,863,000	[1]	350,000	[1]	13,608,000	[1]	1,011,000	[1]
Net proceeds from disposition held by a qualified intermediary in connection wtih Section 1031 exchange					23,285,000		0
Total Property Dispositions [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental income	413,000		473,000		1,359,000		1,419,000
Tenant reimbursements	56,000		55,000		202,000		151,000
Total revenues	469,000		528,000		1,561,000		1,570,000
Property expenses	33,000		43,000		118,000		153,000
Real estate taxes	29,000		32,000		93,000		97,000
Depreciation and amortization	99,000		103,000		297,000		309,000
Total expenses	161,000		178,000		508,000		559,000
Income from discontinued operations before net gain on dispositions of discontinued operations	308,000		350,000		1,053,000		1,011,000
Net gain on dispositions of discontinued operations	12,555,000		0		12,555,000		0
Total income from discontinued operations	12,863,000		350,000		13,608,000		1,011,000
Barnes Canyon Pacific Heights Drive Disposition [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property Type					Office
Month of Disposition	September				September
Number of Buildings	2				2
Rentable Square Feet	90,558				90,558
Sales Price					23,900,000
Net proceeds from disposition held by a qualified intermediary in connection wtih Section 1031 exchange					$ 23,285,000

[1] Income from discontinued operations is from our Reportable Segment - Office Properties.