Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Liability for real estate additions due to Vornado	$ 3,622	$ 233
Development fees \| Vornado
Liability for real estate additions due to Vornado	$ 456	$ 18

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