Debt instrument maturity date

No definition available.

Name:	alx_DebtInstrumentMaturityDateString
Namespace Prefix:	alx_
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The term of the extension available, in years.

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loan that will become recourse in case of a substantial casualty

+ References

No definition available.

+ Details

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

- Definition

The total number of term extensions available to the borrower.

+ References

No definition available.

+ Details

Name:	alx_NumberOfExtensionsAvailable
Namespace Prefix:	alx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of cash mortgage collateralized

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The carrying value, as of the date of each statement of financial position presented, of real estate which is owned and pledged as collateral for related debt obligation

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: 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(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 3

-Subparagraph (SX 210.12-04.(a))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ 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:	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 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9

+ Details

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

- Definition

The net cash inflow or outflow in aggregate debt due to repayments and proceeds from additional borrowings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Period Type:	duration

- Details

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Namespace Prefix:
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Period Type:

- Details

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- Details

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- Details

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Mortgages Payable (Details) (USD $)	0 Months Ended	12 Months Ended
Mortgages Payable (Details) (USD $)	Feb. 28, 2014 Extensions	Dec. 31, 2014	Dec. 31, 2013
Mortgage Loans on Real Estate [Line Items]
Notes Payable		$ 1,032,780,000us-gaap_NotesPayable	$ 1,049,959,000us-gaap_NotesPayable
Net carrying value of real estate collaterizing the debt		706,791,000
Repayments of Long-term Debt [Abstract]
2015		401,439,000
2016		3,440,000
2017		3,707,000
2018		324,194,000
2019		0
Thereafter		300,000,000
Rego Park 1 Shopping [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized		100.00%
Rego Park 1 Shopping [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String		2015-03
Interest rate (in percentage)		0.40%
Notes Payable		78,246,000	78,246,000
Rego Park 2 Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String		2018-11
Interest rate (in percentage)		2.02%
Notes Payable		266,534,000	269,496,000
Basis spread over LIBOR (in percentage)		1.85%
Lexington Avenue Property [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable		320,000,000
Mortgage Loan Converted To Recourse		75,000,000
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Office Space [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2017-03	2021-03
Interest rate (in percentage)		1.11%
Notes Payable		300,000,000	314,217,000
Basis spread over LIBOR (in percentage)	0.95%
Proceeds from (Repayments of) Debt	300,000,000
Number Of Extensions Available	4
Length Of Extension Available	1 year
Derivative Liability Notional Amount	300,000,000
Derivative cap interest rate, LIBOR	6.00%
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String		2015-07
Interest rate (in percentage)		4.93%
Notes Payable		320,000,000	320,000,000
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Retail Space [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loan Converted To Recourse		75,000,000
Paramus Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String		2018-10
Interest rate (in percentage)		2.90%
Notes Payable		$ 68,000,000	$ 68,000,000