Stock-based Compensation (Vornado Stock Options) (Details) - Employee Stock Option	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Shares
Options outstanding at beginning of period (shares) | shares	3,322,069
Granted (shares) | shares	29,867
Exercised (shares) | shares	(449,386)
Cancelled or expired (shares) | shares	(78,650)
Options outstanding at end of period (shares) | shares	2,823,900
Options vested and expected to vest at end of period (shares) | shares	2,881,202
Option exercisable at end of period (shares) | shares	2,762,728
Weighted- Average Exercise Price
Options outstanding at beginning of period (in usd per share) | $ / shares	$ 49.81
Granted (in usd per share) | $ / shares	85.78
Exercised (in usd per share) | $ / shares	62.89
Cancelled or expired (in usd per share) | $ / shares	102.96
Options outstanding at end of period (in usd per share) | $ / shares	46.62
Options vested and expected to vest at end of period (in usd per share) | $ / shares	46.98
Options exercisable at end of period (in usd per share) | $ / shares	$ 45.86
Weighted- Average Remaining Contractual Term
Options outstanding, Weighted Average Remaining Contractual Term	2 years 2 months 12 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	2 years 2 months 12 days
Options exercisable, Weighted Average Remaining Contractual Term	2 years 1 month 6 days
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value | $	$ 89,382,838
Options vested and expected to vest, Aggregate Intrinsic Value | $	90,218,230
Options exercisable, Aggregate Intrinsic Value | $	$ 89,274,127