RENTAL PROPERTIES - Real Estate Detail (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012
Real estate and accumulated depreciation
Encumbrances (First Mortgages)	$ 234,629,675
Initial Cost to Partnerships
Land	57,135,253
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	43,986,579
Gross Amount at Which Carried at Close of Period
Land	57,091,060
Building Improvements	254,860,534
Totals	311,951,597	$ 263,659,293	$ 261,276,898	$ 225,021,946
Accumulated Depreciation	104,797,803	$ 94,196,482	$ 84,579,584	$ 75,905,862
Buildings and improvements
Initial Cost to Partnerships
Building Improvements	$ 210,873,955
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)	$ 39,049,572
Initial Cost to Partnerships
Land	2,112,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	8,836,077
Gross Amount at Which Carried at Close of Period
Land	2,112,000
Building Improvements	17,429,186
Totals	19,541,186
Accumulated Depreciation	11,977,004
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)	2,501,972
Initial Cost to Partnerships
Land	684,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	432,022
Gross Amount at Which Carried at Close of Period
Land	684,000
Building Improvements	3,548,022
Totals	4,232,022
Accumulated Depreciation	2,174,123
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	1,701,168
Gross Amount at Which Carried at Close of Period
Land	177,610
Building Improvements	3,179,527
Totals	3,357,137
Accumulated Depreciation	2,250,924
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	1,196,072
Gross Amount at Which Carried at Close of Period
Land	342,000
Building Improvements	2,563,741
Totals	2,905,741
Accumulated Depreciation	1,765,386
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	3,924,962
Gross Amount at Which Carried at Close of Period
Land	1,410,000
Building Improvements	9,589,778
Totals	10,999,778
Accumulated Depreciation	5,954,341
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	1,286,060
Gross Amount at Which Carried at Close of Period
Land	91,400
Building Improvements	2,026,420
Totals	2,117,820
Accumulated Depreciation	1,225,043
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)	4,460,057
Initial Cost to Partnerships
Land	1,341,737
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	159,257
Gross Amount at Which Carried at Close of Period
Land	1,341,737
Building Improvements	8,616,754
Totals	9,958,491
Accumulated Depreciation	2,314,831
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	249,024
Gross Amount at Which Carried at Close of Period
Land	2,362,596
Building Improvements	4,863,009
Totals	7,225,605
Accumulated Depreciation	1,470,530
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)	36,555,470
Initial Cost to Partnerships
Land	16,054,336
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	581,025
Gross Amount at Which Carried at Close of Period
Land	16,054,336
Building Improvements	45,375,463
Totals	61,429,799
Accumulated Depreciation	15,460,634
Hamilton Green Apartments Residential Apartments Andover, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	44,794,438
Hamilton Linewt LLC Commercial-1031 Exchange Newton, Massachusetts
Initial Cost to Partnerships
Land	884,042
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	138,904
Gross Amount at Which Carried at Close of Period
Land	884,042
Building Improvements	2,791,031
Totals	3,675,073
Accumulated Depreciation	706,988
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	3,281,817
Gross Amount at Which Carried at Close of Period
Land	2,175,000
Building Improvements	15,606,817
Totals	17,781,817
Accumulated Depreciation	9,876,116
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	274,024
Gross Amount at Which Carried at Close of Period
Land	156,000
Building Improvements	908,109
Totals	1,064,109
Accumulated Depreciation	647,937
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	1,585,649
Gross Amount at Which Carried at Close of Period
Land	385,000
Building Improvements	3,125,649
Totals	3,510,649
Accumulated Depreciation	2,245,989
Linhart L.P. Residential/Commercial Newton, Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	1,540,000
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	1,086,013
Gross Amount at Which Carried at Close of Period
Land	1,512,000
Building Improvements	6,787,493
Totals	8,299,493
Accumulated Depreciation	3,702,802
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	2,350,856
Gross Amount at Which Carried at Close of Period
Land	936,000
Building Improvements	6,112,869
Totals	7,048,869
Accumulated Depreciation	4,279,547
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	1,026,852
Gross Amount at Which Carried at Close of Period
Land	46,181
Building Improvements	1,905,175
Totals	1,951,356
Accumulated Depreciation	1,289,709
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	3,946,412
Gross Amount at Which Carried at Close of Period
Land	1,200,000
Building Improvements	8,757,016
Totals	9,957,016
Accumulated Depreciation	5,547,278
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)	20,071,000
Initial Cost to Partnerships
Land	6,247,153
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	825,028
Gross Amount at Which Carried at Close of Period
Land	6,247,153
Building Improvements	25,779,805
Totals	32,026,958
Accumulated Depreciation	3,149,707
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	1,033,453
Gross Amount at Which Carried at Close of Period
Land	72,525
Building Improvements	1,621,230
Totals	1,693,755
Accumulated Depreciation	840,124
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	77,051
Gross Amount at Which Carried at Close of Period
Land	23,346
Building Improvements	267,858
Totals	291,204
Accumulated Depreciation	223,822
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)	14,549,604
Initial Cost to Partnerships
Land	4,686,728
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	(346,294)
Gross Amount at Which Carried at Close of Period
Land	4,686,728
Building Improvements	18,400,617
Totals	23,087,345
Accumulated Depreciation	9,487,028
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)	6,000,000
Initial Cost to Partnerships
Land	3,280,000
Cost Capitalized Subsequent to Acquisition
Improvements, Net of retirements	82,460
Gross Amount at Which Carried at Close of Period
Land	3,280,000
Building Improvements	5,002,460
Totals	8,282,460
Accumulated Depreciation	3,070,591
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	2,342,341
Gross Amount at Which Carried at Close of Period
Land	1,335,000
Building Improvements	9,907,842
Totals	11,242,842
Accumulated Depreciation	6,109,227
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	225,772
Gross Amount at Which Carried at Close of Period
Land	44,965
Building Improvements	4,704,459
Totals	4,749,424
Accumulated Depreciation	2,325,390
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	7,149,052
Gross Amount at Which Carried at Close of Period
Land	417,107
Building Improvements	9,573,688
Totals	9,990,795
Accumulated Depreciation	5,681,385
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	541,522
Gross Amount at Which Carried at Close of Period
Land	9,114,334
Building Improvements	36,416,516
Totals	45,530,850
Accumulated Depreciation	1,021,349
Woodland Park Apartments LLC Newton Massachusetts | Buildings and improvements
Initial Cost to Partnerships
Building Improvements	$ 35,874,994