RENTAL PROPERTIES - Real Estate Detail (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	$ 284,789,713
Initial Cost to Partnerships
Land	75,124,253
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	40,519,055
Gross Amount at Which Carried at Close of Period
Land	75,280,781
Building Improvements	323,349,009
Totals	398,629,793	$ 398,554,000	$ 337,902,411	$ 311,951,597
Accumulated Depreciation	134,019,906	$ 120,190,012	$ 107,391,148	$ 104,797,803
Buildings and improvements
Initial Cost to Partnerships
Building Improvements	$ 282,829,954
Buildings and improvements | Minimum
Gross Amount at Which Carried at Close of Period
Estimated useful lives	10 years
Buildings and improvements | Maximum
Gross Amount at Which Carried at Close of Period
Estimated useful lives	40 years
Other Categories of Assets | Minimum
Gross Amount at Which Carried at Close of Period
Estimated useful lives	5 years
Other Categories of Assets | Maximum
Gross Amount at Which Carried at Close of Period
Estimated useful lives	15 years
Boylston Downtown L.P. Residential Apartments Boston, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	$ 36,718,252
Initial Cost to Partnerships
Land	2,112,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	9,854,811
Gross Amount at Which Carried at Close of Period
Land	2,268,528
Building Improvements	18,447,920
Totals	20,716,448
Accumulated Depreciation	14,573,600
Boylston Downtown L.P. Residential Apartments Boston, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	8,593,109
Brookside Associates LLC Residential Apartments Woburn, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	6,175,000
Initial Cost to Partnerships
Land	684,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	410,411
Gross Amount at Which Carried at Close of Period
Land	684,000
Building Improvements	3,526,411
Totals	4,210,411
Accumulated Depreciation	2,553,664
Brookside Associates LLC Residential Apartments Woburn, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	3,116,000
Clovelly Apartments L.P. Residential Apartments Nashua, New Hampshire
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	4,160,000
Initial Cost to Partnerships
Land	177,610
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,623,654
Gross Amount at Which Carried at Close of Period
Land	177,610
Building Improvements	3,102,013
Totals	3,279,623
Accumulated Depreciation	2,408,585
Clovelly Apartments L.P. Residential Apartments Nashua, New Hampshire | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	1,478,359
Commonwealth 1137 L.P. Residential Apartments Boston, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	3,750,000
Initial Cost to Partnerships
Land	342,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,028,517
Gross Amount at Which Carried at Close of Period
Land	342,000
Building Improvements	2,396,186
Totals	2,738,186
Accumulated Depreciation	1,962,058
Commonwealth 1137 L.P. Residential Apartments Boston, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	1,367,669
Commonwealth 1144 L.P. Residential Apartments Boston, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	14,780,000
Initial Cost to Partnerships
Land	1,410,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	4,216,314
Gross Amount at Which Carried at Close of Period
Land	1,410,000
Building Improvements	9,881,130
Totals	11,291,130
Accumulated Depreciation	7,080,447
Commonwealth 1144 L.P. Residential Apartments Boston, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	5,664,816
Executive Apartments L.P. Residential Apartments Framingham, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	2,415,000
Initial Cost to Partnerships
Land	91,400
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,433,566
Gross Amount at Which Carried at Close of Period
Land	91,400
Building Improvements	2,173,926
Totals	2,265,326
Accumulated Depreciation	1,452,365
Executive Apartments L.P. Residential Apartments Framingham, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	740,360
Hamilton Battle Green LLC Residential Apartments Lexington, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	4,139,001
Initial Cost to Partnerships
Land	1,341,737
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	99,234
Gross Amount at Which Carried at Close of Period
Land	1,341,737
Building Improvements	8,556,731
Totals	9,898,468
Accumulated Depreciation	3,308,558
Hamilton Battle Green LLC Residential Apartments Lexington, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	8,457,497
Hamilton Cypress LLC Commercial-1031 Exchange Brookline, Massachusetts
Initial Cost to Partnerships
Land	2,362,596
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(3,066)
Gross Amount at Which Carried at Close of Period
Land	2,362,596
Building Improvements	4,610,919
Totals	6,973,515
Accumulated Depreciation	1,575,736
Hamilton Cypress LLC Commercial-1031 Exchange Brookline, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	4,613,985
Hamilton Green Apartments Residential Apartments Andover, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	34,525,332
Initial Cost to Partnerships
Land	16,054,336
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(8,407,249)
Gross Amount at Which Carried at Close of Period
Land	16,054,336
Building Improvements	36,387,189
Totals	52,441,525
Accumulated Depreciation	12,108,718
Hamilton Green Apartments Residential Apartments Andover, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	44,794,438
Hamilton Highlands LLC Residential Needham Massachusetts [Member]
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	20,293,723
Initial Cost to Partnerships
Land	6,815,522
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	610,019
Gross Amount at Which Carried at Close of Period
Land	6,815,522
Building Improvements	27,872,106
Totals	34,687,628
Accumulated Depreciation	4,700,854
Hamilton Highlands LLC Residential Needham Massachusetts [Member] | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	27,262,087
Hamilton Linewt LLC Commercial-1031 Exchange Newton, Massachusetts
Initial Cost to Partnerships
Land	884,042
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	134,614
Gross Amount at Which Carried at Close of Period
Land	884,042
Building Improvements	2,786,741
Totals	3,670,783
Accumulated Depreciation	915,144
Hamilton Linewt LLC Commercial-1031 Exchange Newton, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	2,652,127
Hamilton Oaks Associates LLC Residential Apartments Brockton, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	11,925,000
Initial Cost to Partnerships
Land	2,175,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	3,796,076
Gross Amount at Which Carried at Close of Period
Land	2,175,000
Building Improvements	16,121,076
Totals	18,296,076
Accumulated Depreciation	11,851,707
Hamilton Oaks Associates LLC Residential Apartments Brockton, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	12,325,000
Highland Street Apartments, L.P. Residential Apartments Lowell, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	1,050,000
Initial Cost to Partnerships
Land	156,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	347,107
Gross Amount at Which Carried at Close of Period
Land	156,000
Building Improvements	981,192
Totals	1,137,192
Accumulated Depreciation	730,034
Highland Street Apartments, L.P. Residential Apartments Lowell, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	634,085
Linhart L.P. Residential/Commercial Newton, Massachusetts
Initial Cost to Partnerships
Land	385,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,637,532
Gross Amount at Which Carried at Close of Period
Land	385,000
Building Improvements	3,177,532
Totals	3,562,532
Accumulated Depreciation	2,607,882
Linhart L.P. Residential/Commercial Newton, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	1,540,000
Mill Street Gardens Residential Apartments Woburn Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	31,000,000
Initial Cost to Partnerships
Land	9,798,478
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	2,603,900
Gross Amount at Which Carried at Close of Period
Land	9,798,478
Building Improvements	46,172,812
Totals	55,971,290
Accumulated Depreciation	2,945,578
Mill Street Gardens Residential Apartments Woburn Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	43,568,912
Mill Street Development Woburn Massachusetts
Initial Cost to Partnerships
Land	1,375,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	17,280
Gross Amount at Which Carried at Close of Period
Land	1,375,000
Building Improvements	1,142,280
Totals	2,517,280
Accumulated Depreciation	42,150
Mill Street Development Woburn Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	1,125,000
NERA Dean St. Associates LLC Residential Apartments Norwood, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	5,687,000
Initial Cost to Partnerships
Land	1,512,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,148,594
Gross Amount at Which Carried at Close of Period
Land	1,512,000
Building Improvements	6,850,074
Totals	8,362,074
Accumulated Depreciation	4,544,786
NERA Dean St. Associates LLC Residential Apartments Norwood, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	5,701,480
North Beacon 140 L.P. Residential Units Boston, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	6,937,000
Initial Cost to Partnerships
Land	936,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	2,524,593
Gross Amount at Which Carried at Close of Period
Land	936,000
Building Improvements	6,286,606
Totals	7,222,606
Accumulated Depreciation	5,174,097
North Beacon 140 L.P. Residential Units Boston, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	3,762,013
Olde English Apartments L.P. Residential Apartments Lowell, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	3,080,000
Initial Cost to Partnerships
Land	46,181
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	980,124
Gross Amount at Which Carried at Close of Period
Land	46,181
Building Improvements	1,858,447
Totals	1,904,628
Accumulated Depreciation	1,348,781
Olde English Apartments L.P. Residential Apartments Lowell, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	878,323
Redwood Hills L.P. Residential Units Worcester, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	6,743,000
Initial Cost to Partnerships
Land	1,200,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	4,922,230
Gross Amount at Which Carried at Close of Period
Land	1,200,000
Building Improvements	9,732,834
Totals	10,932,834
Accumulated Depreciation	6,874,442
Redwood Hills L.P. Residential Units Worcester, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	4,810,604
Residences at Captain Parkers LLC Residential Apartments Lexington, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	20,750,000
Initial Cost to Partnerships
Land	6,247,153
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	630,086
Gross Amount at Which Carried at Close of Period
Land	6,247,153
Building Improvements	25,584,863
Totals	31,832,016
Accumulated Depreciation	5,874,896
Residences at Captain Parkers LLC Residential Apartments Lexington, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	24,954,777
River Drive L.P. Residential Apartments Danvers, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	3,465,000
Initial Cost to Partnerships
Land	72,525
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	1,047,213
Gross Amount at Which Carried at Close of Period
Land	72,525
Building Improvements	1,634,990
Totals	1,707,515
Accumulated Depreciation	969,388
River Drive L.P. Residential Apartments Danvers, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	587,777
Riverside Apartments Condominium Units Massachusetts
Initial Cost to Partnerships
Land	23,346
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	122,173
Gross Amount at Which Carried at Close of Period
Land	23,346
Building Improvements	312,980
Totals	336,326
Accumulated Depreciation	253,659
Riverside Apartments Condominium Units Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	190,807
School St Assoc LLC Residential Apartments Framingham, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	13,662,721
Initial Cost to Partnerships
Land	4,686,728
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(257,413)
Gross Amount at Which Carried at Close of Period
Land	4,686,728
Building Improvements	18,489,498
Totals	23,176,226
Accumulated Depreciation	11,353,669
School St Assoc LLC Residential Apartments Framingham, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	18,746,911
WRF Associates LLC Strip Mall Framingham, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	6,083,684
Initial Cost to Partnerships
Land	3,280,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	72,337
Gross Amount at Which Carried at Close of Period
Land	3,280,000
Building Improvements	4,992,337
Totals	8,272,337
Accumulated Depreciation	3,568,520
WRF Associates LLC Strip Mall Framingham, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	4,920,000
WCB Associates LLC Residential Apartments Brockton, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	7,000,000
Initial Cost to Partnerships
Land	1,335,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	2,347,295
Gross Amount at Which Carried at Close of Period
Land	1,335,000
Building Improvements	9,912,796
Totals	11,247,796
Accumulated Depreciation	7,340,602
WCB Associates LLC Residential Apartments Brockton, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	7,565,501
Westgate Apartments Residential Units Burlington Massachusetts Member
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	2,500,000
Initial Cost to Partnerships
Land	44,965
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	279,767
Gross Amount at Which Carried at Close of Period
Land	44,965
Building Improvements	4,758,454
Totals	4,803,419
Accumulated Depreciation	2,864,555
Westgate Apartments Residential Units Burlington Massachusetts Member | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	4,478,687
Westgate Apartments LLC Residential Apartments Woburn, Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	15,700,000
Initial Cost to Partnerships
Land	461,300
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	6,314,899
Gross Amount at Which Carried at Close of Period
Land	461,300
Building Improvements	8,739,535
Totals	9,200,835
Accumulated Depreciation	5,726,798
Westgate Apartments LLC Residential Apartments Woburn, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	2,424,636
Woodland Park Apartments LLC Newton Massachusetts
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	22,250,000
Initial Cost to Partnerships
Land	9,114,334
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	984,437
Gross Amount at Which Carried at Close of Period
Land	9,114,334
Building Improvements	36,859,431
Totals	45,973,765
Accumulated Depreciation	7,308,633
Woodland Park Apartments LLC Newton Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	$ 35,874,994