SUBSEQUENT EVENTS (Details)	2 Months Ended	12 Months Ended			149 Months Ended
	Mar. 08, 2020 USD ($) $ / item shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2019 USD ($) $ / item shares	Feb. 07, 2020 USD ($)	Dec. 20, 2019 USD ($)
Subsequent events
Number of depository receipts repurchased | shares			57,803		1,423,109
Repurchase price of depository receipts (in dollars per receipt) | $ / item					28.31
Total cost of repurchase		$ 4,050,137		$ 42,548	$ 44,324,000
Units required to be repurchased (in shares) | shares		2,408		23
57 Mill Street
Subsequent events
Mortgage amount							$ 31,000,000
Subsequent event
Subsequent events
Number of depository receipts repurchased | shares	3,469
Repurchase price of depository receipts (in dollars per receipt) | $ / item	62.01
Price per unit of depository receipts | $ / item	1,860.30
Total cost of repurchase	$ 215,113
Term of debt						15 years
Mortgage amount						$ 6,175,000
Interest rate (as a percent)						3.53%
Loan balance						$ 2,390,000
Class B
Subsequent events
Number of depository receipts repurchased | shares					3,530
Repurchase price of depository receipts (in dollars per receipt) | $ / item					1,024.08
Total cost of repurchase		$ 769,458		$ 8,084
Class B | Subsequent event
Subsequent events
Value of units required to be repurchased	51,087
General Partnership
Subsequent events
Number of depository receipts repurchased | shares					186
Total cost of repurchase		$ 40,498	$ 40,498	$ 426
General Partnership | Subsequent event
Subsequent events
Value of units required to be repurchased	$ 2,689