Segment information (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2020 USD ($) business leases tenants	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) business leases number_of_properties tenants	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | number_of_properties					4
Assets	$ 4,731,062,000				$ 4,731,062,000		$ 4,770,560,000
Operating Leases, Income Statement, Lease Revenue	114,831,000		$ 105,049,000		338,271,000	$ 298,569,000
Financing Revenues	11,271,000		14,710,000		38,115,000	44,675,000
miscellaneous revenues	266,000		0		812,000	0
adjusted funds from operations	84,304,000		80,348,000		259,059,000	234,686,000
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	(3,602,000)		(2,137,000)		(199,075,000)	(6,094,000)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	26,746,000		26,086,000		(102,321,000)	75,093,000
Interest Expense	29,879,000		28,799,000		90,608,000	83,166,000
Depreciation	41,867,000		37,805,000		123,943,000	110,647,000
Share-based Compensation	152,000		0		(5,463,000)	0
Gain (Loss) on Condemnation	0		(591,000)		0	(1,316,000)
Gains (Losses) on Sales of Investment Real Estate	(3,261,000)		0		(3,261,000)	0
Gains (Losses) on Sales of Investment Real Estate	0		0		0	(4,000)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0		0		0	(747,000)
Gain (Loss) on Extinguishment of Debt	518,000		15,000		6,674,000	84,000
Trading Securities, Unrealized Holding Gain	(49,000)		0		(528,000)	0
loan fees received					882,000	1,347,000
noncash loan interest income	577,000		818,000		179,828,000	1,143,000
Management fees net of deferrals	582,000		305,000		1,058,000	922,000
Loans and Leases Receivable, Allowance	0		5,530,000		1,963,000	16,144,000
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions	120,000		0		120,000	0
rental and other property revenues	115,160,000		105,493,000		342,149,000	300,886,000
Revenues	$ 126,697,000		120,203,000		$ 381,076,000	345,561,000
Number Of Retail Tenants | tenants	900				900
Number Of Leases In Bankruptcy Proceedings | leases	11				11
Number Of Companies With Leases From Company In Bankruptcy Proceedings | business	6				6
Payments to Acquire Productive Assets	$ 12,144,000		10,673,000		$ 37,953,000	29,120,000
Contract with Customer, Liability	800,000				800,000
Contract with Customer, Liability, Revenue Recognized		$ 2,800,000		$ 2,800,000
Multifamily Communities
Segment Reporting Information [Line Items]
Assets	2,146,061,000				2,146,061,000		2,047,905,000
adjusted funds from operations	33,267,000		30,191,000		103,414,000	90,825,000
Interest Expense	15,739,000		16,108,000		46,537,000	46,729,000
Depreciation	26,516,000		23,869,000		77,745,000	74,304,000
financingsegment [Member]
Segment Reporting Information [Line Items]
Assets	354,130,000				354,130,000		409,226,000
adjusted funds from operations	11,245,000		14,710,000		38,089,000	44,675,000
Interest Expense	902,000		360,000		3,939,000	1,697,000
New Market Properties [Member]
Segment Reporting Information [Line Items]
adjusted funds from operations	19,235,000		18,211,000		57,381,000	50,441,000
Interest Expense	6,539,000		6,422,000		19,876,000	18,123,000
Depreciation	12,688,000		11,677,000		39,410,000	32,644,000
rental and other property revenues	26,707,000		25,270,000		80,815,000	70,211,000
Payments to Acquire Productive Assets	2,017,000		2,699,000		$ 4,557,000	5,703,000
Average Recurring Rental Revenue Collections, After Rent Deferrrals					1.00%
Retail Segment [Member]
Segment Reporting Information [Line Items]
Assets	1,074,584,000				$ 1,074,584,000		1,125,230,000
Office Building
Segment Reporting Information [Line Items]
Assets	1,130,962,000				1,130,962,000		1,123,212,000
adjusted funds from operations	20,291,000		17,236,000		59,363,000	48,745,000
Interest Expense	6,699,000		5,909,000		20,256,000	16,617,000
Depreciation	12,590,000		10,693,000		35,941,000	30,243,000
Contract with Customer, Liability		$ 36,900,000						$ 47,000,000.0
Other Assets [Member]
Segment Reporting Information [Line Items]
Assets	25,325,000				25,325,000		$ 64,987,000
All Other Segments [Member]
Segment Reporting Information [Line Items]
adjusted funds from operations	7,898,000		1,370,000		23,113,000	4,197,000
Miscellaneous
Segment Reporting Information [Line Items]
adjusted funds from operations	266,000		0		812,000	0
Residential Properties
Segment Reporting Information [Line Items]
rental and other property revenues	60,643,000		56,008,000		180,325,000	162,669,000
Payments to Acquire Productive Assets	4,348,000		4,709,000		10,938,000	11,377,000
Preferred Office Properties
Segment Reporting Information [Line Items]
rental and other property revenues	27,810,000		24,215,000		81,009,000	68,006,000
Payments to Acquire Productive Assets	$ 5,779,000		$ 3,265,000		$ 22,458,000	$ 12,040,000