XML 69 R53.htm IDEA: XBRL DOCUMENT
v3.20.4

Stock-Based Compensation - Additional Information (Detail) - 2016 Omnibus Stock Plan - USD ($) | 1 Months Ended |
--- | --- | ---
May 31, 2020 | Dec. 31, 2020
Director |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |
Non-option equity instruments granted per director (in shares) | 329 |
Non option equity instruments market value | $ 75,000 |
Non-option equity instruments grant date fair value per grant | 56,250 |
Non-option equity instruments grant date fair value per grant total | $ 450,000 |
Non-option equity instruments, outstanding, number (in shares) |  | 14,916
Shares available for future grant under the plan (in shares) |  | 490,871
Newly Appointed Director |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |
Additional non option equity instruments granted per director (in shares) | 876 |
Additional non option equity instruments market value | $ 200,000 |
Additional non option equity instruments grant date fair value total | $ 150,000 |

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Additional Non Option Equity Instruments Grant Date Fair Value Total + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalNonOptionEquityInstrumentsGrantDateFairValueTotal
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Additional Non Option Equity Instruments Granted Per Director + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalNonOptionEquityInstrumentsGrantedPerDirector
Namespace Prefix: | alx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Additional Non Option Equity Instruments Market Value + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalNonOptionEquityInstrumentsMarketValue
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Share based compensation arrangement by share based payment award non option equity instruments grant date fair value per grant. + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantDateFairValuePerGrant
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Grant Date Fair Value Total + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantDateFairValueTotal
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of restricted stock units granted to each of the members of the board of directors during the period. + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantedPerDirector
Namespace Prefix: | alx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Market Value + References No definition available. + Details
Name: | alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsMarketValue
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of equity instruments other than options outstanding, including both vested and non-vested instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_PlanNameAxis=alx_A2016OmnibusStockPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=srt_DirectorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=alx_NewlyAppointedDirectorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |