ORGANIZATION AND PRINCIPAL ACTIVITIES - Summary of principal subsidiaries and VIEs (Detail)	12 Months Ended
Dec. 31, 2019
Subsidiaries [Member] \| Jiayin Holdings Limited
Date of incorporation/establishment or acquisition	Jan. 31, 2018
Place of incorporation/establishment	BVI
Percentage of legal ownership	100.00%
Principal activities	Investment Holding
Subsidiaries [Member] \| Geerong(HK) Limited (formerly known as "Jiayin (HK) Limited")
Date of incorporation/establishment or acquisition	Jan. 31, 2018
Place of incorporation/establishment	Hong Kong
Percentage of legal ownership	100.00%
Principal activities	Investment Holding
Subsidiaries [Member] \| Jiayin Southeast Asia Holdings Limited
Date of incorporation/establishment or acquisition	Feb. 28, 2018
Place of incorporation/establishment	BVI
Percentage of legal ownership	100.00%
Principal activities	Investment Holding
Subsidiaries [Member] \| Shanghai Kunjia Technology Co., Ltd.
Date of incorporation/establishment or acquisition	Jun. 30, 2018
Place of incorporation/establishment	Shanghai
Percentage of legal ownership	100.00%
Principal activities	Investment Holding
Subsidiaries [Member] \| Geerong Yunke Information Technology Co., Ltd
Date of incorporation/establishment or acquisition	Jul. 30, 2019
Place of incorporation/establishment	Shanghai
Percentage of legal ownership	100.00%
Principal activities	Technology development and consumer finance services
Subsidiaries [Member] \| Geerong Yun (Shanghai) Enterprise Development Co., Ltd.
Date of incorporation/establishment or acquisition	Sep. 30, 2019
Place of incorporation/establishment	Shanghai
Percentage of legal ownership	100.00%
Principal activities	Technology development and consumer finance services
Variable Interest Entity, Primary Beneficiary [Member] \| Shanghai Jiayin Finance Technology Co., Ltd.
Date of incorporation/establishment or acquisition	Jun. 30, 2015
Place of incorporation/establishment	Shanghai
Percentage of legal ownership	100.00%
Principal activities	Technology service
Variable Interest Entity, Primary Beneficiary [Member] \| Shanghai Niwodai Internet Finance Information Services Co., Ltd.
Date of incorporation/establishment or acquisition	Sep. 30, 2015
Place of incorporation/establishment	Shanghai
Percentage of legal ownership	100.00%
Principal activities	Technology development and consumer finance services

Date when an entity was incorporated

No definition available.

Name:	dei_EntityIncorporationDateOfIncorporation
Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Entity Incorporation, State Country Name

+ References

No definition available.

+ Details

Name:	jfin_EntityIncorporationStateorCountryName
Namespace Prefix:	jfin_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions.

+ References

No definition available.

+ Details

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

- Definition

Description of the major products or services that a subsidiary or equity investee sells or provides and its principal markets, including locations of those markets.

+ References

No definition available.

+ Details

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

- Definition

Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=SL6759159-111685

+ Details

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

- Definition

The overall reason that the Variable Interest Entity (VIE) was formed. For example, the VIE was formed to hold financial assets, including loans or receivables, real estate or other property or to engage in research and development or other activities on behalf of another company.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5747-111685

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 16
-Subparagraph (c,d)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=SL6228967-111685

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=SL6759159-111685

+ Details

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

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_JiayinHoldingsLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_GeerongHKLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_JiayinSoutheastAsiaHoldingsLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_ShanghaiKunjiaTechnologyCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_GeerongyunkeInformationTechnologyCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=jfin_GeerongYunShanghaiEnterpriseDevelopmentCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=jfin_ShanghaiJiayinFinanceTechnologyCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=jfin_ShanghaiNiwodaiInternetFinanceInformationServicesCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: