Accounting for the Impairment or Disposal of Long-Lived Assets (Assets Held for Sale) (Details) ft² in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($) ft² bldg	Dec. 31, 2018 USD ($) a bldg	Dec. 31, 2017 USD ($) a bldg	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		60
Gross proceeds		$ 849,528	$ 379,139	$ 1,192,918
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Assets held for sale		220,545	622,575
Liabilities held for sale		$ 3,183	$ 14,623
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		49
Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		10	27
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements		$ 92,889	$ 155,974
Buildings and improvements		126,001	493,615
Development in progress		0	45,035
Land held for development		0	0
Accumulated depreciation		(36,569)	(187,335)
Deferred financing and leasing costs, net		2,781	15,296
Other assets		3,575	19,351
Assets held for sale		188,677	541,936
Liabilities held for sale		$ 3,042	$ 13,470
Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		37	2
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		11
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		2	8
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		1
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements		$ 1,301	$ 3,476
Buildings and improvements		5,638	80,738
Development in progress		0	0
Land held for development		26,253	863
Accumulated depreciation		(1,546)	(11,785)
Deferred financing and leasing costs, net		58	2,210
Other assets		164	5,137
Assets held for sale		31,868	80,639
Liabilities held for sale		$ 141	$ 1,153
Held for Sale, Includes Disposal Groups and Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		11	25
Area of land (in acres) | a		69	9
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements		$ 94,190	$ 159,450
Buildings and improvements		131,639	574,353
Development in progress		0	45,035
Land held for development		26,253	863
Accumulated depreciation		(38,115)	(199,120)
Deferred financing and leasing costs, net		2,839	17,506
Other assets		3,739	24,488
Assets held for sale		220,545	622,575
Liabilities held for sale		$ 3,183	$ 14,623
Disposed of by Sale, Discontinued Operations and Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		39	10
Florida | Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		3
Florida | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		0	0
Houston | Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		0
Houston | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg		0	1
Subsequent Event | Florida | Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings (in buildings) | bldg	3
Area of real estate property (in sqft) | ft²	151
Gross proceeds	$ 23,400