Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Discontinued operations [Abstract]
Rental and other revenues	$ 365	$ 6,051	$ 1,624	$ 12,389
Operating expenses:
Rental property and other expenses	100	2,171	405	4,306
Depreciation and amortization	81	1,607	559	3,426
Impairments of real estate assets	0	0	415	0
Total operating expenses	181	3,778	1,379	7,732
Interest expense	0	123	0	248
Income from discontinued operations	184	2,150	245	4,409
Net gains on disposition of discontinued operations	13,163	1,385	13,694	6,519
Total discontinued operations	13,347	3,535	13,939	10,928
Assets, held for sale [Abstract]
Land	0		0		8,450
Buildings and tenant improvements	0		0		50,663
Land held for development	0		0		2,368
Less accumulated depreciation	0		0		(17,969)
Net real estate assets	0		0		43,512
Accrued straight line rents receivable	0		0		1,962
Deferred leasing costs, net	0		0		2,004
Prepaid expenses and other assets	0		0		30
Real estate and other assets, net, held for sale	$ 0		$ 0		$ 47,508
Orlando, FL Office Properties Disposition (3/2013) [Member]
Assets, held for sale [Abstract]
Number of office properties included in real estate and other assets, net, held for sale					2
Atlanta, GA Industrial Properties Held For Sale (3/2013) [Member]
Assets, held for sale [Abstract]
Number of industrial properties included in real estate and other assets, net, held for sale					13