Impairment and Disposal of Long-Lived Assets Assets Held for Sale (Details) $ in Thousands	Mar. 31, 2018 USD ($) bldg	Dec. 31, 2017 USD ($) bldg
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Assets held for sale	$ 238,199	$ 330,186
Liabilities held for sale	$ 1,849	9,503
Properties sold or classified as held for sale in the prior year and current year to date [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	41
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	2
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 3,199	3,476
Buildings and improvements	80,738	80,738
Development in progress	0	0
Land held for development	0	863
Accumulated depreciation	(11,760)	(11,785)
Deferred financing and leasing costs, net	2,210	2,210
Other assets	5,265	5,137
Assets held for sale	79,652	80,639
Liabilities held for sale	$ 332	1,153
Discontinued Operations, Held-for-sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	5
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 20,404	41,191
Buildings and improvements	141,416	248,514
Development in progress	0	45,035
Land held for development	0	0
Accumulated depreciation	(15,318)	(105,786)
Deferred financing and leasing costs, net	6,964	10,280
Other assets	5,081	10,313
Assets held for sale	158,547	249,547
Liabilities held for sale	$ 1,517	8,350
Held for Sale, Includes Disposal Groups and Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	7
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 23,603	44,667
Buildings and improvements	222,154	329,252
Development in progress	0	45,035
Land held for development	0	863
Accumulated depreciation	(27,078)	(117,571)
Deferred financing and leasing costs, net	9,174	12,490
Other assets	10,346	15,450
Assets held for sale	238,199	330,186
Liabilities held for sale	$ 1,849	$ 9,503
Discontinued Operations, Disposed of by Sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	23	2
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	30
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	1	8
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	11
Disposed of by Sale, Discontinued Operations and Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	24	10