RENTAL PROPERTIES - Real Estate Detail (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	$ 283,219,855
Initial Cost to Partnerships
Land	75,124,253
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	40,599,790
Gross Amount at Which Carried at Close of Period
Land	75,124,253
Building Improvements	323,429,744
Totals	398,554,000	$ 337,902,411	$ 311,951,597	$ 263,659,293
Accumulated Depreciation	120,190,012	$ 107,391,148	$ 104,797,803	$ 94,196,482
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)	$ 37,526,355
Initial Cost to Partnerships
Land	2,112,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	9,472,008
Gross Amount at Which Carried at Close of Period
Land	2,112,000
Building Improvements	18,065,117
Totals	20,177,117
Accumulated Depreciation	13,711,733
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)	2,393,123
Initial Cost to Partnerships
Land	684,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	434,650
Gross Amount at Which Carried at Close of Period
Land	684,000
Building Improvements	3,550,650
Totals	4,234,650
Accumulated Depreciation	2,437,790
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	1,630,368
Gross Amount at Which Carried at Close of Period
Land	177,610
Building Improvements	3,108,727
Totals	3,286,337
Accumulated Depreciation	2,343,274
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	1,032,076
Gross Amount at Which Carried at Close of Period
Land	342,000
Building Improvements	2,399,745
Totals	2,741,745
Accumulated Depreciation	1,881,160
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	4,080,451
Gross Amount at Which Carried at Close of Period
Land	1,410,000
Building Improvements	9,745,267
Totals	11,155,267
Accumulated Depreciation	6,793,741
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	1,455,692
Gross Amount at Which Carried at Close of Period
Land	91,400
Building Improvements	2,196,052
Totals	2,287,452
Accumulated Depreciation	1,416,671
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)	4,251,347
Initial Cost to Partnerships
Land	1,341,737
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	137,011
Gross Amount at Which Carried at Close of Period
Land	1,341,737
Building Improvements	8,594,508
Totals	9,936,245
Accumulated Depreciation	2,993,941
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	(15,711)
Gross Amount at Which Carried at Close of Period
Land	2,362,596
Building Improvements	4,598,274
Totals	6,960,870
Accumulated Depreciation	1,436,449
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)	35,233,819
Initial Cost to Partnerships
Land	16,054,336
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(8,468,758)
Gross Amount at Which Carried at Close of Period
Land	16,054,336
Building Improvements	36,325,680
Totals	52,380,016
Accumulated Depreciation	10,519,175
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)	20,570,836
Initial Cost to Partnerships
Land	6,815,522
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	483,597
Gross Amount at Which Carried at Close of Period
Land	6,815,522
Building Improvements	27,745,684
Totals	34,561,206
Accumulated Depreciation	2,961,729
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	842,583
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	3,690,689
Gross Amount at Which Carried at Close of Period
Land	2,175,000
Building Improvements	16,015,689
Totals	18,190,689
Accumulated Depreciation	11,136,328
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	346,681
Gross Amount at Which Carried at Close of Period
Land	156,000
Building Improvements	980,766
Totals	1,136,766
Accumulated Depreciation	699,783
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	1,613,782
Gross Amount at Which Carried at Close of Period
Land	385,000
Building Improvements	3,153,782
Totals	3,538,782
Accumulated Depreciation	2,493,281
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	2,500,000
Gross Amount at Which Carried at Close of Period
Land	9,798,478
Building Improvements	46,068,912
Totals	55,867,390
Accumulated Depreciation	84,419
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	1,875
Gross Amount at Which Carried at Close of Period
Land	1,375,000
Building Improvements	1,126,875
Totals	2,501,875
Accumulated Depreciation	1,211
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	1,144,138
Gross Amount at Which Carried at Close of Period
Land	1,512,000
Building Improvements	6,845,618
Totals	8,357,618
Accumulated Depreciation	4,280,075
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	2,516,151
Gross Amount at Which Carried at Close of Period
Land	936,000
Building Improvements	6,278,164
Totals	7,214,164
Accumulated Depreciation	4,899,010
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	926,901
Gross Amount at Which Carried at Close of Period
Land	46,181
Building Improvements	1,805,224
Totals	1,851,405
Accumulated Depreciation	1,286,893
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	4,787,863
Gross Amount at Which Carried at Close of Period
Land	1,200,000
Building Improvements	9,598,467
Totals	10,798,467
Accumulated Depreciation	6,443,200
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	2,015,170
Gross Amount at Which Carried at Close of Period
Land	6,247,153
Building Improvements	26,969,947
Totals	33,217,100
Accumulated Depreciation	6,071,499
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	1,032,211
Gross Amount at Which Carried at Close of Period
Land	72,525
Building Improvements	1,619,988
Totals	1,692,513
Accumulated Depreciation	913,256
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	96,682
Gross Amount at Which Carried at Close of Period
Land	23,346
Building Improvements	287,489
Totals	310,835
Accumulated Depreciation	248,364
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)	13,968,691
Initial Cost to Partnerships
Land	4,686,728
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(369,769)
Gross Amount at Which Carried at Close of Period
Land	4,686,728
Building Improvements	18,377,142
Totals	23,063,870
Accumulated Depreciation	10,582,758
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	3,399,768
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	2,493,490
Gross Amount at Which Carried at Close of Period
Land	1,335,000
Building Improvements	10,058,991
Totals	11,393,991
Accumulated Depreciation	7,064,497
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	269,798
Gross Amount at Which Carried at Close of Period
Land	44,965
Building Improvements	4,748,485
Totals	4,793,450
Accumulated Depreciation	2,686,999
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	6,201,555
Gross Amount at Which Carried at Close of Period
Land	461,300
Building Improvements	8,626,191
Totals	9,087,491
Accumulated Depreciation	5,366,389
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	884,238
Gross Amount at Which Carried at Close of Period
Land	9,114,334
Building Improvements	36,759,232
Totals	45,873,566
Accumulated Depreciation	5,194,036
Woodland Park Apartments LLC Newton Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	$ 35,874,994