Realized and Unrealized Investment Gains and Losses (Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Realized and Unrealized Investment Gains (Losses) [Abstract]
Fixed maturity securities, Gains	$ 39,113	$ 48,860	$ 34,295
Fixed maturity securities, Losses	(4,420)	(14,670)	(6,436)
Equity securities available for sale	38,296	70,235	97,300
Investment funds	96,204	10,976	74,777
Gain (Loss) on Sale of Properties	85,659us-gaap_GainLossOnSaleOfProperties	0us-gaap_GainLossOnSaleOfProperties	0us-gaap_GainLossOnSaleOfProperties
Other gains	0	12,185	1,515
Net realized gains on investments sales	254,852	127,586	201,451
Other-than-temporary impairments	0us-gaap_ImpairmentOfInvestments	(6,042)us-gaap_ImpairmentOfInvestments	(4,984)us-gaap_ImpairmentOfInvestments
Decrease in valuation allowance	0	0	13,998
Net other-than-temporary impairments	0	(6,042)	9,014
Net investment gains	254,852	121,544	210,465
Income tax expense	(89,198)	(47,426)	(73,663)
Total after-tax investment gains	$ 165,654	$ 74,118	$ 136,802

Amount of increase (decrease) of allowance reserve for credit loss.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) on sale or disposal of an equity method investment.

+ References

No definition available.

+ Details

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

- Definition

The difference between the book value and the sale price of other nonspecific investments. This element is used when other, more specific, elements are not appropriate. This element refers to the gain (loss) included in earnings.

+ References

+ Details

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

- Definition

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

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

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

+ Details

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

- Definition

Available for Sale and Held-to-Maturity Realized Gains, Debt Securities, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

+ Details

Name:	wrb_AvailableforSaleandHeldtoMaturityRealizedGainsDebtSecuritiesExcludingOtherthanTemporaryImpairmentsandAllowances
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Available for Sale and Held-to-Maturity Realized Losses, Debt Securities, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

+ Details

Name:	wrb_AvailableforSaleandHeldtoMaturityRealizedLossesDebtSecuritiesExcludingOtherthanTemporaryImpairmentsandAllowances
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Available-for-sale Securities, Gross Realized Gain (Loss), Equity Securities, Excluding Other than Temporary Impairments

+ References

No definition available.

+ Details

Name:	wrb_AvailableforsaleSecuritiesGrossRealizedGainLossEquitySecuritiesExcludingOtherthanTemporaryImpairments
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Investments, Realized Loss, Other than Temporary Impairments and Allowances, Amount

+ References

No definition available.

+ Details

Name:	wrb_InvestmentsRealizedLossOtherthanTemporaryImpairmentsandAllowancesAmount
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	wrb_RealizedAndUnrealizedInvestmentGainsLossesAbstract
Namespace Prefix:	wrb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Realized Gain (Loss) on Investments, Including Equity Method Investments, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

+ Details

Name:	wrb_RealizedGainLossonInvestmentsIncludingEquityMethodInvestmentsExcludingOtherthanTemporaryImpairmentsandAllowances
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Realized Gain (Loss) on Marketable Securities, Equity Method Investments, and Other Investments, Before Tax

+ References

No definition available.

+ Details

Name:	wrb_RealizedGainLossonMarketableSecuritiesEquityMethodInvestmentsandOtherInvestmentsBeforeTax
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Realized Gain (Loss) on Marketable Securities, Equity Method Investments, and Other Investments, Net of Tax

+ References

No definition available.

+ Details

Name:	wrb_RealizedGainLossonMarketableSecuritiesEquityMethodInvestmentsandOtherInvestmentsNetofTax
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Realized Gain (Loss) on Marketable Securities, Equity Method Investments, and Other Investments, Tax

+ References

No definition available.

+ Details

Name:	wrb_RealizedGainLossonMarketableSecuritiesEquityMethodInvestmentsandOtherInvestmentsTax
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration