Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems
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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Name:	us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Current year monetary amount of the change in worker's compensation reserve liability resulting from the discount applied to reduce the reserve to present value.

Reference 1: 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),13(b))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

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Period Type:	instant

Favorable Reserve Development Net Of Premium Offsets

No definition available.

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Incurred but Not Reported (IBNR) Claims, Amount

No definition available.

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Reported Claims, Amount

No definition available.

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Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)

No definition available.

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance

No definition available.

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Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
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Unfavorable Reserve Development Net Of Premium Offsets

No definition available.