Notes payable gross.

No definition available.

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Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_ParamusPropertyMember
Namespace Prefix:
Data Type:	na
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Mortgages Payable - Summary of Outstanding Mortgages Payable (Detail) - USD ($) $ in Thousands		5 Months Ended	9 Months Ended
	Jun. 01, 2017	May 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Notes payable			$ 1,240,069	$ 1,052,359
Mortgages [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage payable gross			1,253,393	1,056,147
Deferred debt issuance costs, net of accumulated amortization of $4,995 and $6,824 respectively			(13,324)	(3,788)
Notes payable			$ 1,240,069	1,052,359
Mortgages [Member] \| Rego Park I [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Maturity date			Mar. 12, 2018
Interest rate (in percentage)			0.35%
Mortgage payable gross			$ 78,246	78,246
Mortgages [Member] \| Rego Park II [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Maturity date			Nov. 30, 2018
Interest rate (in percentage)			3.09%
Mortgage payable gross			$ 257,147	259,901
Mortgages [Member] \| 731 Lexington Avenue [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Maturity date			Aug. 05, 2022
Interest rate (in percentage)			2.63%
Mortgage payable gross			$ 350,000	350,000
Mortgages [Member] \| 731 Lexington Avenue [Member] \| Office Space [Member]
Mortgage Loans on Real Estate [Line Items]
Maturity date	Jun. 11, 2024	Mar. 11, 2021	Jun. 11, 2024
Interest rate (in percentage)			2.14%
Mortgage payable gross			$ 500,000	300,000
Notes payable	$ 500,000	$ 300,000
Mortgages [Member] \| Paramus [Member]
Mortgage Loans on Real Estate [Line Items]
Maturity date			Oct. 05, 2018
Interest rate (in percentage)			2.90%
Mortgage payable gross			$ 68,000	$ 68,000