FAIR VALUE MEASUREMENTS - Fair value Assets and Liabilities Transfer Amount (Details) - USD ($)	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
FAIR VALUE MEASUREMENTS
Asset transfers from level1 to level2	$ 0	$ 0
Asset transfers from level2 to level1	0	0
Liability transfers from level1 to level2	0	0
Liability transfers from level2 to level1	0	0
Asset transfers into or out of level 3	0	0
Liability transfers into or out of level 3	$ 0	$ 0

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

No definition available.

Name:	atch_FairValueAssetsLevel1ToLevel2TransfersAmount1
Namespace Prefix:	atch_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

No definition available.

Name:	atch_FairValueAssetsLevel2ToLevel1TransfersAmount1
Namespace Prefix:	atch_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

No definition available.

Name:	atch_FairValueLiabilitiesLevel1ToLevel2TransfersAmount1
Namespace Prefix:	atch_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

No definition available.

Name:	atch_FairValueLiabilitiesLevel2ToLevel1TransfersAmount1
Namespace Prefix:	atch_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

No definition available.

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

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

No definition available.

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

Amount of transfers of financial instrument classified as a liability into (out of) level 3 of the fair value hierarchy.

No definition available.

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration