Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016
Liability for real estate additions due to Vornado	$ 115	$ 1,401
Development Fees [Member] \| Vornado [Member]
Liability for real estate additions due to Vornado	$ 27	$ 74

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