Impairment and Disposal of Long-Lived Assets Discontinued Operations (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) bldg	Mar. 31, 2017 USD ($)	Dec. 31, 2017 bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain on property dispositions	$ (90,049,000)	$ 0
Net income	94,333,000	2,896,000
Income available to common shareholders	$ 92,138,000	2,828,000
Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	5
Discontinued Operations, Disposed of by Sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	23		2
Discontinued Operations [Member]
Discontined Operations Income Statement [Line Items]
Capital expenditures	$ 274,000	7,300,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	8,179,000	9,600,000
Operating expenses	1,972,000	3,641,000
Depreciation and amortization	1,219,000	2,368,000
Interest and other income	5,000	14,000
Interest expense	709,000	709,000
Net income before gain on property dispositions	4,284,000	2,896,000
Gain on property dispositions	90,049,000	0
Net income	94,333,000	2,896,000
Noncontrolling interest - operating partnership	2,195,000	68,000
Income available to common shareholders	$ (92,138,000)	$ (2,828,000)
Arizona [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	5
Southeastern PA [Member] | Discontinued Operations, Disposed of by Sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	23