Represents prepaid reinsurance premiums ceded as percentage of total balance.

No definition available.

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Namespace Prefix:	jrvr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents reinsurance recoverables on unpaid losses percentage of total balance.

+ References

No definition available.

+ Details

Name:	jrvr_ReinsuranceRecoverablesOnUnpaidLossesPercentageOfTotalBalance
Namespace Prefix:	jrvr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

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

- Definition

The unexpired portion of premiums ceded on policies in force as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6485982&loc=d3e22818-158507

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99380617&loc=SL75241803-196195

+ Details

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

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

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

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to and paid by the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),6)
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

+ Details

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

- Details

Name:	us-gaap_CededCreditRiskAxis=jrvr_CreditRiskReinsurerOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CededCreditRiskAxis=jrvr_CreditRiskReinsurerTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CededCreditRiskAxis=jrvr_CreditRiskReinsurerThreeMember
Namespace Prefix:
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Period Type:

Reinsurance - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 668,045	$ 467,371	$ 302,524
Reinsurance recoverable on paid losses	$ 33,221	18,344
Percentage of reinsurance recoverables on unpaid losses for 3 largest reinsurers	46.30%
Prepaid reinsurance premiums ceded	$ 178,976	$ 112,498
Percentage of prepaid reinsurance premiums ceded for 3 largest reinsurers	41.30%
Largest reinsurer one
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 191,400
Prepaid reinsurance premiums ceded	27,200
Largest reinsurer two
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	59,600
Prepaid reinsurance premiums ceded	24,500
Largest reinsurer three
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	58,000
Prepaid reinsurance premiums ceded	$ 22,300