Discontinued Operations Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Revenue:
Rents	$ 115,552	$ 109,699
Tenant reimbursements	20,357	18,696
Termination fees	496	1,490
Other Real Estate Revenue	967	1,512
Total revenue	140,608	134,539
Expenses:
Property operating expenses	39,641	38,077
Real estate taxes	14,430	13,567
Depreciation and amortization	49,861	48,096
Total operating expenses	111,908	107,040
Interest income	58	483
Income from discontinued operations before gain on sale of interests in real estate	618	2,527
Net gain on disposition of discontinued operations	5,304	14,668
Income from discontinued operations	5,922	17,195
Discontinued Operations [Member]
Revenue:
Rents	2,818	8,650
Tenant reimbursements	323	727
Termination fees		7
Other Real Estate Revenue	20	23
Total revenue	3,161	9,407
Expenses:
Property operating expenses	1,089	2,487
Real estate taxes	332	1,036
Depreciation and amortization	1,122	3,357
Total operating expenses	2,543	6,880
Income from discontinued operations before gain on sale of interests in real estate	618	2,527
Net gain on disposition of discontinued operations	5,304	14,668
Income from discontinued operations	$ 5,922	$ 17,195