SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of movement of contract assets (Detail) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Opening balance	¥ 242,881	¥ 881,296
Addition in the current year	67,278	1,236,968
Collection in the current year	(170,261)	(1,618,631)
Write off in the current year	(87,362)	(256,752)
Transfer out due to disposal of Shanghai Caiyin	(190,491)
Reclass to refund liability	(137,955)
Ending balance	0	242,881
Current loan products
Opening balance	54,615	335,550
Addition in the current year	41,747	898,142
Collection in the current year	(41,134)	(1,001,265)
Write off in the current year	(46,306)	(177,812)
Transfer out due to disposal of Shanghai Caiyin	(140,988)
Reclass to refund liability	(132,066)
Ending balance	0	54,615
Other online standard loan products
Opening balance	14,102	162,942
Addition in the current year	3,827	26,987
Collection in the current year	(10,398)	(107,384)
Write off in the current year	(5,382)	(68,443)
Transfer out due to disposal of Shanghai Caiyin	(6,492)
Reclass to refund liability	(4,343)
Ending balance	0	14,102
Offline and non-standard loan products
Opening balance	163,026	376,304
Addition in the current year	21,704	41,548
Collection in the current year	(107,591)	(244,329)
Write off in the current year	(35,674)	(10,497)
Transfer out due to disposal of Shanghai Caiyin	(43,011)
Reclass to refund liability	(1,546)
Ending balance	0	163,026
Other services
Opening balance	11,138	6,500
Addition in the current year		270,291
Collection in the current year	(11,138)	(265,653)
Transfer out due to disposal of Shanghai Caiyin	0
Ending balance	¥ 0	¥ 11,138

Accounts receivable allowance for credit loss transfers.

No definition available.

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Namespace Prefix:	jfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract with customer asset reclassification to refund liability.

+ References

No definition available.

+ Details

Name:	jfin_ContractWithCustomerAssetReclassificationToRefundLiability
Namespace Prefix:	jfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of writeoff of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

+ Details

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

- Definition

Amount, before allowance for credit loss, of right to consideration in exchange for good or service transferred to customer, when right is conditioned on something other than passage of time.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130533-203044

+ Details

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

- Definition

Amount of increase from purchase of right to consideration in exchange for good or service transferred to customer when right is conditioned on passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919244-210447

+ Details

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

- Definition

Amount of decrease from sale of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time and other income.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919244-210447

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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- Details

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- Details

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