Impairment and Disposal of Long-Lived Assets Discontinued Operations (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) bldg	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) bldg	Jun. 30, 2017 USD ($)	Dec. 31, 2017 bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
(Loss) gain on property dispositions	$ (238,000)	$ 0	$ 89,811,000	$ 0
Net income	7,670,000	7,516,000	105,295,000	13,872,000
Income from discontinued operations available to common shareholders	$ 7,491,000	7,340,000	$ 102,842,000	13,547,000
Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	14		14
Discontinued Operations, Disposed of by Sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	23		23		2
Discontinued Operations [Member]
Discontined Operations Income Statement [Line Items]
Expected timing of disposal			one year
Capital expenditures	$ 53,000	13,200,000	$ 2,900,000	31,900,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	12,412,000	19,672,000	28,162,000	38,214,000
Operating expenses	(2,648,000)	(6,784,000)	(6,409,000)	(13,434,000)
Depreciation and amortization	(441,000)	(4,107,000)	(3,273,000)	(8,087,000)
Interest and other income	8,000	20,000	20,000	43,000
Interest expense	(1,423,000)	(1,285,000)	(3,016,000)	(2,864,000)
Net income before (loss) gain on property dispositions	7,908,000	7,516,000	15,484,000	13,872,000
(Loss) gain on property dispositions	(238,000)	0	89,811,000	0
Net income	7,670,000	7,516,000	105,295,000	13,872,000
Noncontrolling interest - operating partnership	(179,000)	(176,000)	(2,453,000)	(325,000)
Income from discontinued operations available to common shareholders	$ 7,491,000	$ 7,340,000	$ 102,842,000	$ 13,547,000
Arizona [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	5		5
Southeastern PA [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	8		8
DC Metro [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	1		1