Fixed Maturity Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Jun. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,187,207	$ 854,391
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	2,326,935	1,274,605
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,698,944	$ 1,294,721
Noninvestment Grade Investments at Loss Position
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	33
Aggregate Fair Value	$ 156,735
Gross Unrealized Loss	$ 7,106
Noninvestment Grade Investments at Loss Position \| Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	12
Aggregate Fair Value	$ 51,383
Gross Unrealized Loss	$ 2,773
Noninvestment Grade Investments at Loss Position \| Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	4
Aggregate Fair Value	$ 3,477
Gross Unrealized Loss	$ 28
Noninvestment Grade Investments at Loss Position \| Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	13
Aggregate Fair Value	$ 97,985
Gross Unrealized Loss	$ 4,199
Noninvestment Grade Investments at Loss Position \| Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities \| position	3
Aggregate Fair Value	$ 268
Gross Unrealized Loss	$ 104
Noninvestment Grade Investments at Loss Position \| State and municipal
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	1
Aggregate Fair Value	$ 3,622
Gross Unrealized Loss	$ 2

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
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

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

- Definition

Available For Sale Equity Securities Continuous Unrealized Loss Position Aggregate Loss

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentSecondaryCategorizationAxis=wrb_NoninvestmentGradeInvestmentsatLossLessthan5MillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ForeignGovernmentDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: