Leases - Lessor Narrative (Details) ft² in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 28, 2019 ft²	Dec. 31, 2016 USD ($)
Lessor, Lease, Description [Line Items]
Net income	$ 14,434,000	$ 16,493,000	$ 11,283,000	$ 17,865,000	$ 9,971,000	$ 15,003,000	$ 17,570,000	$ (9,700,000)	$ 60,075,000	$ 32,844,000	$ 80,509,000
Allowance for doubtful accounts	0				671,000				0	671,000	1,501,000		$ 1,473,000
Revenues	$ 55,880,000	$ 57,760,000	$ 55,932,000	$ 56,778,000	$ 57,567,000	$ 59,125,000	$ 58,253,000	$ 57,880,000	$ 226,350,000	232,825,000	230,574,000
Minimum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, term (years)	5 years								5 years
Maximum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, term (years)	25 years								25 years
The Alexander apartment tower | Minimum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, term (years)	1 year								1 year
The Alexander apartment tower | Maximum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, term (years)	2 years								2 years
Bloomberg | 731 Lexington Avenue
Lessor, Lease, Description [Line Items]
Additional area of property leased (in square feet) | ft²												49
Revenue Benchmark | Bloomberg | Customer Concentration Risk
Lessor, Lease, Description [Line Items]
Revenues									$ 109,113,000	$ 107,356,000	$ 105,224,000
Percentage rent contributed by tenant									48.00%	46.00%	46.00%
Accounting Standards Update 2016-02
Lessor, Lease, Description [Line Items]
Net income									$ (209,000)