Impairment and Disposal of Long-Lived Assets Assets Held for Sale (Details) $ in Thousands	Jun. 30, 2018 USD ($) bldg	Dec. 31, 2017 USD ($) bldg
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Assets held for sale	$ 376,551	$ 547,668
Liabilities held for sale	$ 2,521	11,178
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	50
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | 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	0	863
Accumulated depreciation	(1,546)	(11,785)
Deferred financing and leasing costs, net	58	2,210
Other assets	193	5,137
Assets held for sale	5,644	80,639
Liabilities held for sale	$ 164	1,153
Discontinued Operations, Held-for-sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	14
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 112,171	132,957
Buildings and improvements	310,414	417,459
Development in progress	0	45,035
Land held for development	0	0
Accumulated depreciation	(70,116)	(158,955)
Deferred financing and leasing costs, net	10,397	14,168
Other assets	8,041	16,365
Assets held for sale	370,907	467,029
Liabilities held for sale	$ 2,357	10,025
Held for Sale, Includes Disposal Groups and Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	15
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 113,472	136,433
Buildings and improvements	316,052	498,197
Development in progress	0	45,035
Land held for development	0	863
Accumulated depreciation	(71,662)	(170,740)
Deferred financing and leasing costs, net	10,455	16,378
Other assets	8,234	21,502
Assets held for sale	376,551	547,668
Liabilities held for sale	$ 2,521	$ 11,178
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	39
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	2	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	25	10