Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=123594786&loc=SL75136599-209740

Name:	us-gaap_EquitySecuritiesFvNi
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Period Type:	instant

Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- References

No definition available.

Name:	us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract
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Data Type:	xbrli:stringItemType
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Period Type:	duration

- References

No definition available.

Name:	us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract
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Data Type:	xbrli:stringItemType
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Period Type:	duration

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

Name:	us-gaap_LoansReceivableFairValueDisclosure
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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

No definition available.

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
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Period Type:	instant

Fair value portion of liabilities pertaining to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include, but are not limited to, short positions in securities, derivatives and commodities.

No definition available.

Name:	us-gaap_TradingLiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Name:	us-gaap_TradingSecurities
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Arbitrage Trading Accounts Payable To Brokers And Clearing Organizations

No definition available.

Name:	wrb_ArbitrageTradingAccountsPayableToBrokersAndClearingOrganizations
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
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Due From Brokers, Fair Value Disclosure

No definition available.

Name:	wrb_DueFromBrokersFairValueDisclosure
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Due To Broker Fair Value

No definition available.

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

Marketable Securities, Fair Value Disclosure

No definition available.