Stock-based Compensation (Performance Conditioned AO LTIP Units & Vornado Stock Options Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds received from exercise of employee share options and other	$ 6,903	$ 7,309	$ 29,712
Vornado stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	4 years
Stock options expiration term	10 years
Grants in period, weighted average grant date fair value (usd per share)	$ 16.64	$ 18.42	$ 25.84
Proceeds received from exercise of employee share options and other	$ 5,495	$ 5,927	$ 28,253
Exercises in period, intrinsic value	$ 18,954	25,820	$ 9,178
Performance Conditioned AO LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period, weighted average grant date fair value (usd per share)	$ 64.48
Threshold percentage to satisfy performance based condition	110.00%
Threshold period to satisfy performance based condition	20 days
Share-based compensation expense	$ 7,481
Share-based payment arrangement, amount capitalized	1,502
Grant-date fair value	$ 8,983
AO LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options expiration term	10 years
Threshold level, percent	100.00%
Special allocation, percent	10.00%
Special distribution, percent	10.00%
Grant-date fair value	$ 3,429	$ 3,484

Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of cost capitalized for award under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The fair value at grant date for equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Share-based Compensation Arrangement by Share-based Payment Award, Special Allocation

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Share-based Compensation Arrangement by Share-based Payment Award, Threshold Level

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