Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Jun. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 4,162,130	$ 5,160,888
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	713,497	734,969
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	1,499,781	1,212,529
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 653,903	$ 1,100,392
Noninvestment Grade Investments at Loss Position
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	15
Aggregate Fair Value	$ 22,115
Gross Unrealized Loss	$ 834
Noninvestment Grade Investments at Loss Position \| Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	3
Aggregate Fair Value	$ 9,655
Gross Unrealized Loss	$ 250
Noninvestment Grade Investments at Loss Position \| Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	6
Aggregate Fair Value	$ 6,239
Gross Unrealized Loss	$ 215
Noninvestment Grade Investments at Loss Position \| Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities \| position	4
Aggregate Fair Value	$ 3,299
Gross Unrealized Loss	$ 137
Noninvestment Grade Investments at Loss Position \| Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	2
Aggregate Fair Value	$ 2,922
Gross Unrealized Loss	$ 232

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=73527792&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=73527792&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_AssetBackedSecuritiesMember
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_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: