Property and Equipment - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Property Plant and Equipment [Line Items]
Depreciation and amortization on property and equipment	$ 16.7	$ 11.4	$ 30.6	$ 20.5
Capital leases assets, gross carrying values	3.9		3.9		$ 3.5
Capital leases assets, accumulated amortization balances	1.9		1.9		1.3
GPO Assets	16.3		16.3		18.4
Property and Equipment
Property Plant and Equipment [Line Items]
Depreciation and amortization on property and equipment	14.6	$ 10.1	28.4	$ 19.2
Rental Fleet
Property Plant and Equipment [Line Items]
Capital leases assets, accumulated amortization balances	$ 377.7		$ 377.7		$ 285.1

Capital leases lessee balance sheet assets by major class accumulated amortization.

No definition available.

Name:	altg_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedAmortization
Namespace Prefix:	altg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

GPO assets.

+ References

No definition available.

+ Details

Name:	altg_GPOAssets
Namespace Prefix:	altg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84164758&loc=d3e45014-112735

+ Details

Name:	us-gaap_CapitalLeasedAssetsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_PropertyPlantAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=altg_RentalFleetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: