Commitments, Litigation And Contingent Liabilities (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Commitment to Invest in Certain Investment Funds	$ 406.2
Commitment to Invest in Real Estate Construction Projects	$ 359.7

Commitment to Invest in Real Estate Construction Projects

No definition available.

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