Realized And Unrealized Investment Gains Realized and Unrealized Investment Gains (Loss) (Realized and unrealized investmetns gains(losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Fixed maturity securities, gains	$ 5,605	$ 23,064
Fixed maturity securities, losses	(3,965)	(2,940)
Equity securities available for sale	42,707	125
Investment funds	1,267	(460)
Real Estate Realized Gain (Loss) on Disposal	3,300	5,024
Other	3,434	644
Realized Gain (Loss) on Investments, Including Equity Method Investments, Excluding Other than Temporary Impairments and Allowances	52,348	25,457
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	0	(18,114)
Gain (Loss) on Investments	52,348	7,343
Net investment gains	52,348	7,343
Income tax expense	(18,322)	(2,570)
After-tax net realized investment gains	$ 34,026	$ 4,773

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

No definition available.

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

- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (c)-(e)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Reference 2: 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=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c),9(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27405-111563

+ Details

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

- Definition

Amount of gain (loss) included in earnings for investments classified as other.

+ 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=66902209&loc=d3e3602-108585

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
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

REal Estate Realized Gain (Loss) on Disposal

+ References

No definition available.

+ Details

Name:	wrb_REalEstateRealizedGainLossonDisposal
Namespace Prefix:	wrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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