Stock-based Compensation (Summary of Share Based Compensation Expense) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ (2,578)	$ (2,578)	$ (2,578)	$ 53,908	$ 31,722	$ 32,829
OP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				39,969	17,763	20,630
Performance Conditioned AO LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				8,263	0	0
AO LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				2,636	2,113	0
OPPs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				1,944	10,689	10,723
Vornado restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				549	570	729
Vornado stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense				$ 547	$ 587	$ 747