Stock-based Compensation (AO LTIP Units Vornado Stock Options) (Details)	12 Months Ended
Dec. 31, 2018 USD ($) $ / shares shares
AO LTIP Units
Shares
Options outstanding at beginning of period (shares) \| shares	185,046
Cancelled or expired (shares) \| shares	(6,200)
Options outstanding at end of period (shares) \| shares	178,846
Weighted- Average Exercise Price
Options outstanding at beginning of period (in usd per share) \| $ / shares	$ 72.40
Cancelled or expired (in usd per share) \| $ / shares	72.40
Options outstanding at end of period (in usd per share) \| $ / shares	$ 72.40
Employee Stock Options
Shares
Options outstanding at beginning of period (shares) \| shares	2,823,900
Granted (shares) \| shares	33,897
Exercised (shares) \| shares	(620,157)
Cancelled or expired (shares) \| shares	(7,347)
Options outstanding at end of period (shares) \| shares	2,230,293
Options vested and expected to vest at end of period (shares) \| shares	2,240,526
Option exercisable at end of period (shares) \| shares	2,162,843
Weighted- Average Exercise Price
Options outstanding at beginning of period (in usd per share) \| $ / shares	$ 46.62
Granted (in usd per share) \| $ / shares	72.40
Exercised (in usd per share) \| $ / shares	28.52
Cancelled or expired (in usd per share) \| $ / shares	75.25
Options outstanding at end of period (in usd per share) \| $ / shares	51.95
Options vested and expected to vest at end of period (in usd per share) \| $ / shares	52.13
Options exercisable at end of period (in usd per share) \| $ / shares	$ 51.15
Weighted- Average Remaining Contractual Term
Options outstanding, Weighted Average Remaining Contractual Term	1 year 219 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	1 year 219 days
Options exercisable, Weighted Average Remaining Contractual Term	1 year 146 days
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value \| $	$ 26,464,877
Options vested and expected to vest, Aggregate Intrinsic Value \| $	26,472,765
Options exercisable, Aggregate Intrinsic Value \| $	$ 26,464,877

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Number of options outstanding, including both vested and non-vested options.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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