Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Liability for real estate additions due to Vornado	$ 3,209	$ 829
Development fees \| Vornado
Liability for real estate additions due to Vornado	$ 146	$ 24

Non-cash additions to real estate included in accounts payable and accrued expenses.

No definition available.

Name:	alx_NonCashAdditionsToRealEstate
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration