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, 2018 USD ($) ft² bldg	Dec. 31, 2017 USD ($) a bldg	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		37
Area of land | a		9
Gross proceeds		$ 379,139	$ 1,192,918	$ 530,440
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Disposal Group, Including Discontinued Operation, Assets		182,228	164,888
Liabilities held for sale		$ 4,551	8,079
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 properties | bldg		27
Discontinued Operations, Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		25
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Disposal Group, Including Discontinued Operations, Land and Land Improvements		$ 25,848	34,406
Disposal Group, Including Discontinued Operations, Building and Improvements		163,195	217,833
Disposal Group, Including Discontinued Operation, Land Available for Development		0	0
Disposal Group, Including Discontinued Operations, Accumulated Depreciation		(98,346)	(106,933)
Disposal Group, Including Discontinued Operation, Intangible Assets		3,502	6,355
Disposal Group, Including Discontinued Operation, Other Assets		7,390	8,679
Disposal Group, Including Discontinued Operation, Assets		101,589	160,340
Liabilities held for sale		$ 3,398	8,079
Discontinued Operations, Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		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 properties | bldg		10
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		2
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Disposal Group, Including Discontinued Operations, Land and Land Improvements		$ 3,476	0
Disposal Group, Including Discontinued Operations, Building and Improvements		80,738	0
Disposal Group, Including Discontinued Operation, Land Available for Development		863	4,548
Disposal Group, Including Discontinued Operations, Accumulated Depreciation		(11,785)	0
Disposal Group, Including Discontinued Operation, Intangible Assets		2,210	0
Disposal Group, Including Discontinued Operation, Other Assets		5,137	0
Disposal Group, Including Discontinued Operation, Assets		80,639	4,548
Liabilities held for sale		$ 1,153	0
Disposal Group, 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 properties | bldg		8
Held for Sale, Includes Disposal Groups and Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		27
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Disposal Group, Including Discontinued Operations, Land and Land Improvements		$ 29,324	34,406
Disposal Group, Including Discontinued Operations, Building and Improvements		243,933	217,833
Disposal Group, Including Discontinued Operation, Land Available for Development		863	4,548
Disposal Group, Including Discontinued Operations, Accumulated Depreciation		(110,131)	(106,933)
Disposal Group, Including Discontinued Operation, Intangible Assets		5,712	6,355
Disposal Group, Including Discontinued Operation, Other Assets		12,527	8,679
Disposal Group, Including Discontinued Operation, Assets		182,228	164,888
Liabilities held for sale		$ 4,551	$ 8,079
Disposed of by Sale, Discontinued Operations and Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		10
Southeastern PA [Member] | Discontinued Operations, Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		25
Southeastern PA [Member] | Discontinued Operations, Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		1
Houston [Member] | Discontinued Operations, Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg		1
Subsequent Event [Member] | Southeastern PA [Member] | Discontinued Operations, Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties | bldg	14
Area of Real Estate Property | ft²	641
Gross proceeds	$ 76,900