Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) $ in Thousands	3 Months Ended
Mar. 31, 2025 USD ($)
Development fees	$ 2,188
Vornado
Development fees	$ 419

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