Impairment and Disposal of Long-Lived Assets Assets Held for Sale (Details) $ in Thousands	Sep. 30, 2018 USD ($) bldg	Dec. 31, 2017 USD ($) bldg
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Assets held for sale	$ 294,191	$ 610,611
Liabilities held for sale	$ 3,986	14,282
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	54
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	10,227	863
Accumulated depreciation	(1,546)	(11,785)
Deferred financing and leasing costs, net	58	2,210
Other assets	165	5,137
Assets held for sale	15,843	80,639
Liabilities held for sale	$ 191	1,153
Discontinued Operations, Held-for-sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	13
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 111,517	152,708
Buildings and improvements	222,983	471,628
Development in progress	0	45,035
Land held for development	0	0
Accumulated depreciation	(67,821)	(172,604)
Deferred financing and leasing costs, net	4,433	14,914
Other assets	7,236	18,291
Assets held for sale	278,348	529,972
Liabilities held for sale	$ 3,795	13,129
Held for Sale, Includes Disposal Groups and Discontinued Operations [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	$ 112,818	156,184
Buildings and improvements	228,621	552,366
Development in progress	0	45,035
Land held for development	10,227	863
Accumulated depreciation	(69,367)	(184,389)
Deferred financing and leasing costs, net	4,491	17,124
Other assets	7,401	23,428
Assets held for sale	294,191	610,611
Liabilities held for sale	$ 3,986	$ 14,282
Discontinued Operations, Disposed of by Sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	28	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	43
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	30	10