Consolidated Statements of Cash Flows (Parentheticals) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Cash paid for interest, capitalized	$ 459us-gaap_InterestPaidCapitalized	$ 30us-gaap_InterestPaidCapitalized
Non Cash Additions To Real Estate	13,717alx_NonCashAdditionsToRealEstate	1,750alx_NonCashAdditionsToRealEstate
Vornado [Member]
Non Cash Additions To Real Estate	$ 4,159

The amount of cash paid during the current period for interest that is capitalized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

Name:	us-gaap_InterestPaidCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration