Interest rate cap and swap contracts (Details) - USD ($)			1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 07, 2018	Dec. 07, 2017	Sep. 29, 2016	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Oct. 31, 2017	Apr. 22, 2016	Dec. 26, 2012
Derivative [Line Items]
Mortgages and construction loan payable				$ 352,623,000		$ 352,623,000		$ 323,435,000
Interest rate swap contract assets				3,807,000		3,807,000		1,600,000
Unrealized gain (loss) on derivatives				19,000		19,000
Net unrealized gain (loss) on interest rate swap contracts				907,000	$ (240,000)	2,538,000	$ 2,606,000
Interest rate swap contract liabilities								439,000
Damascus Centre Swap [Member]
Derivative [Line Items]
Interest rate swap contract assets				883,000		883,000
Unrealized gain (loss) on derivatives								2,952,000
Wayne PSC swap [Member]
Derivative [Line Items]
Interest rate swap contract assets				2,313,000		2,313,000
Regency Swap [Member]
Derivative [Line Items]
Interest rate swap contract assets				238,000		238,000
Monmouth swap [Member]
Derivative [Line Items]
Interest rate swap contract assets				266,000		266,000
Wells Fargo Bank [Member]
Derivative [Line Items]
Refinanced loan amount	$ 115,300,000
Loan amount	118,500,000
Leasing costs	$ 3,380,000
Basis points, interest rate	2.85%
Maturity date of loan	Feb. 06, 2021
Grande Rotunda LLC [Member]
Derivative [Line Items]
Loan amount				118,500,000		118,500,000
Unrealized gain (loss) on derivatives				19,000		19,000
Increase (Decrease) in Derivative Assets and Liabilities						107,000
Grande Rotunda LLC Construction Loan [Member]
Derivative [Line Items]
Loan amount				121,900,000		121,900,000
Notional amount of interest rate swap				$ 121,900,000		$ 121,900,000
Fixed interest rate				3.00%		3.00%
Maturity date of loan						Mar. 05, 2020
Provident Bank [Member]
Derivative [Line Items]
Loan amount		$ 12,350,000		$ 16,200,000		$ 16,200,000
Notional amount of interest rate swap		$ 12,350,000		$ 16,100,000		$ 16,100,000
Fixed interest rate		4.35%		3.75%		3.75%
Basis points, interest rate		1.80%		2.75%		1.25%
Maturity date of loan		Dec. 15, 2027				Dec. 15, 2024
Wayne PSC, LLC Loan [Member]
Derivative [Line Items]
Refinanced loan amount			$ 24,200,000
Loan amount				$ 24,800,000		$ 24,800,000
Description of loan amendment terms			In order to minimize interest rate volatility during the term of the loan, Wayne PSC, LLC entered into an interest rate swap agreement that, in effect, converted the floating interest rate to a fixed interest rate of 3.625% over the term of the loan.
Notional amount of interest rate swap				24,800,000		24,800,000
Fixed interest rate			3.625%
Basis points, interest rate			2.20%
People's United Bank [Member]
Derivative [Line Items]
Loan amount			$ 25,800,000	20,100,000		20,100,000
Mortgages and construction loan payable									$ 2,320,000
Notional amount of interest rate swap				$ 20,200,000		$ 20,200,000
People's United Bank [Member] | Tranche One [Member]
Derivative [Line Items]
Loan amount										$ 20,000,000
Fixed interest rate				3.81%		3.81%
Basis points, interest rate						2.10%
Maturity date of loan						Jan. 03, 2023
People's United Bank [Member] | Tranche Two [Member]
Derivative [Line Items]
Fixed interest rate				3.53%		3.53%
Damascus Centre [Member]
Derivative [Line Items]
Interest rate swap contract assets								275,000
Wayne PSC swap [Member]
Derivative [Line Items]
Interest rate swap contract assets								1,325,000
Regency Swap [Member]
Derivative [Line Items]
Interest rate swap contract liabilities								$ 439,000
Station Place on Monmouth, LLC [Member]
Derivative [Line Items]
Percentage of acquisition		100.00%