Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenue:
Rents													$ 480,600	$ 461,101	$ 467,188
Tenant reimbursements													81,236	77,139	76,652
Termination fees													2,993	5,576	3,601
Other													5,440	4,310	3,328
Total revenue	146,609	144,308	144,094	[1]	146,794	[2]	146,084	140,179	134,191	[1]	139,502	[2]	581,805	559,956	567,920
Expenses:
Property operating expenses													171,991	167,911	162,940
Real estate taxes													56,230	53,564	56,224
Depreciation and amortization													217,680	210,592	203,572
Total operating expenses													476,093	461,239	451,553
Income from discontinued operations before gain on sale of interests in real estate													1,253	2,659	6,960
Net gain on disposition of discontinued operations													7,264	11,011	1,241
Provision for impairment													0	0	(3,700)
Income from discontinued operations													8,517	13,670	4,501
Discontinued Operations [Member]
Revenue:
Rents													3,779	9,363	19,095
Tenant reimbursements													1,198	3,678	8,453
Termination fees													33	190	0
Other													(50)	70	238
Total revenue													4,960	13,301	27,786
Expenses:
Property operating expenses													1,717	4,641	9,884
Real estate taxes													559	1,816	3,769
Depreciation and amortization													1,431	4,185	7,173
Total operating expenses													$ 3,707	$ 10,642	$ 20,826

[1] The consolidated statement of operations for the second quarter of 2011 also contained an out of period depreciation and amortization expense adjustment of $4.7 million relating to intangible assets representing tenant relationships and in-place leases that should have been written off in prior periods. This resulted in the overstatement of depreciation and amortization expense by $4.7 million million in the current year. During the year ended December 31, 2010, depreciation and amortization expense was overstated by $1.7 million and was understated by $1.4 million, $1.8 million, $1.7 million and $1.5 million during the years ended December 31, 2009, 2008, 2007, and 2006, respectively. As management believes that this error was not material to prior years' consolidated financial statements and that the impact of recording the error in the current year is not material to the Company's consolidated financial statements, the Company recorded the related adjustment during the current year.

[2] During the first quarter of 2011, the Company recorded additional income of $0.5 million related to electricity charges in prior years that were under-billed to a certain tenant. This resulted in the overstatement of total revenue by $0.5 million during the current year and in the understatement of total revenue by $0.3 million and $0.2 million for the years ended December 31, 2009 and 2008, respectively. As management believes that this error was not material to prior years' consolidated financial statements and that the impact of recording the error in the current year is not material to the Company's consolidated financial statements, the Company recorded the related adjustment in the current year.