Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Assets
Fixed maturity securities	$ 11,826,928	$ 11,312,037
Fixed maturity securities, fair value	11,848,850	11,330,910
Equity securities available for sale	561,785	443,439
Equity securities available for sale, fair value	561,785	443,439
Arbitrage trading account	419,249	397,312
Arbitrage trading account, fair value	419,249	397,312
Loans receivable	325,073	263,187
Loans receivable, fair value	316,204	245,169
Cash and cash equivalents	1,012,220	911,742	794,881	642,952
Cash and cash equivalents, fair value	1,012,220	911,742
Trading account receivables from brokers and clearing organizations	275,844	318,240
Trading account receivables from brokers and clearing organizations, fair value	275,844	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	40,520	62,514
Trading account securities sold but not yet purchased, fair value	40,520	62,514
Due to broker	38,213	0
Due to broker, fair value	38,213	0
Junior subordinated debentures	243,102	242,997
Junior subordinated debentures, fair value	244,400	258,400
Senior notes and other debt	1,865,380	1,500,503
Senior notes and other debt, fair value	$ 2,040,509	$ 1,587,473

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+ Details

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