Mortgages Payable (Narrative) (Details) - USD ($)		12 Months Ended
	Dec. 11, 2018	Dec. 31, 2018	Dec. 12, 2018	Oct. 03, 2018	Oct. 02, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Mortgages payable, net of deferred debt issuance costs		$ 1,161,534,000				$ 1,240,222,000
Refinance date		May 11, 2018
Repayment date		Jun. 06, 2018
Paramus
Debt Instrument [Line Items]
Interest rate					2.90%
Matures in October 2021 | Paramus
Debt Instrument [Line Items]
Mortgages payable, net of deferred debt issuance costs				$ 68,000,000
Interest rate		4.72%
Matures in December 2025 | Rego Park II
Debt Instrument [Line Items]
Mortgages payable, net of deferred debt issuance costs			$ 252,544,000
Interest rate		3.87%
Matures in December 2025 | Rego Park II | LIBOR
Debt Instrument [Line Items]
Basis spread over LIBOR		1.35%
Matures in January 2019 | Rego Park II | LIBOR
Debt Instrument [Line Items]
Basis spread over LIBOR	1.85%