Commitments, Litigation And Contingent Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Significant Purchase Commitment Remaining Maximum Amount Committed	$ 167

Significant Purchase Commitment Remaining Maximum Amount Committed

No definition available.

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