Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Assets
Fixed maturity securities	$ 11,892,412	$ 11,312,037
Fixed maturity securities, fair value	11,916,722	11,330,910
Equity securities available for sale	425,322	443,439
Equity securities available for sale, fair value	425,322	443,439
Arbitrage trading account	417,446	397,312
Arbitrage trading account, fair value	417,446	397,312
Loans receivable	371,408	263,187
Loans receivable, fair value	363,875	245,169
Cash and cash equivalents	1,189,564	911,742	866,130	642,952
Cash and cash equivalents, fair value	1,189,564	911,742
Trading account receivables from brokers and clearing organizations	387,352	318,240
Trading account receivables from brokers and clearing organizations, fair value	387,352	318,240
Due from broker	0	10,875
Due from broker fair value	0	10,875
Liabilities
Trading account securities sold but not yet purchased	149,405	62,514
Trading account securities sold but not yet purchased, fair value	149,405	62,514
Due to broker	82,746	0
Due to broker, fair value	82,746	0
Junior subordinated debentures	243,154	242,997
Junior subordinated debentures, fair value	254,800	258,400
Senior notes and other debt	1,877,431	1,500,503
Senior notes and other debt, fair value	$ 2,110,492	$ 1,587,473

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+ Details

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