Consolidated Statement of Changes in Equity (Parentheticals) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Statement of Stockholders' Equity [Abstract]
Special long-term capital gain dividend	$ 623,178

Special dividend paid to distribute the long term capital gain from the sale of real estate

No definition available.

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