Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2011
Summary of Significant Accounting Policies [Abstract]
Principles of consolidation and basis of presentation	Principles of consolidation and basis of presentation The consolidated financial statements, which include the accounts of W. R. Berkley Corporation and its subsidiaries (the "Company"), have been prepared on the basis of U.S. generally accepted accounting principles ("GAAP"). All significant intercompany transactions and balances have been eliminated. Reclassifications have been made in the 2010 and 2009 financial statements to conform to the presentation of the 2011 financial statements. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the revenues and expenses reflected during the reporting period. The most significant items on our balance sheet that involve a greater degree of accounting estimates that are subject to change in the future are the valuation of investments, other than temporary impairments, loss and loss adjustment expense reserves and premium estimates. Actual results could differ from those estimates.
Revenue recognition	Revenue recognition Insurance premiums are recognized as written at the inception of the policy. Reinsurance premiums are estimated based upon information received from ceding companies, and subsequent differences from such estimates are recorded in the period they are determined. Insurance and reinsurance premiums are primarily earned on a pro rata basis over the policy term. Fees for services are earned over the period that the services are provided. Audit premiums are recognized when they are reliably determinable. The change in accruals for earned but unbilled audit premiums increased net premiums written and premiums earned by $1 million in 2011, and decreased net premiums written and premiums earned by $7 million and $23 million in 2010 and 2009, respectively. Revenues from wholly-owned investees are derived from services provided to the general aviation market, including fuel and line service, aircraft sales and maintenance, avionics and engineering services and parts fabrication. Revenue is recognized upon delivery of aircraft, delivery of fuel, shipment of parts and upon completion of services.
Cash and cash equivalents	Cash and cash equivalents Cash equivalents consist of funds invested in money market accounts and investments with an effective maturity of three months or less when purchased.
Investments	Investments Fixed maturity securities classified as available for sale are carried at estimated fair value, with unrealized gains and losses, net of applicable income taxes, excluded from earnings and reported as a component of comprehensive income and a separate component of stockholders' equity. Fixed maturity securities that the Company has the positive intent and ability to hold to maturity are classified as held to maturity and reported at amortized cost. Investment income from fixed maturity securities is recognized based on the constant effective yield method. Premiums and discounts on mortgage-backed securities are adjusted for the effects of actual and anticipated prepayments on a retrospective basis. Equity securities classified as available for sale are carried at estimated fair value, with unrealized gains and losses, net of applicable income taxes, excluded from earnings and reported as a component of comprehensive income and a separate component of stockholders' equity. Equity securities that the Company purchased with the intent to sell in the near-term are classified as trading account securities and are reported at estimated fair value. Realized and unrealized gains and losses from trading activity are reported as net investment income and are recorded at the trade date. Short sales and short call options are presented as trading securities sold but not yet purchased. Unsettled trades and the net margin balances held by the clearing broker are presented as a trading account receivable from brokers and clearing organizations. Investment funds are carried under the equity method of accounting. For certain investment funds, the Company's share of the earnings or losses is reported on a one-quarter lag in order to facilitate the timely completion of the Company's consolidated financial statements. Loans receivable represent commercial real estate mortgage loans and bank loans and are carried at amortized cost. The Company monitors the performance of its loans receivable and establishes an allowance for loan losses for loans where the Company determines it is probable that the contractual terms will not be met, with a corresponding charge to earnings. For loans that are evaluated individually and deemed to be impaired, the Company establishes a specific allowance based on a discounted cash flow analysis and comparable cost and sales methodologies, if appropriate. Individual loans that are not considered impaired and smaller-balance homogeneous loans are evaluated collectively and a general allowance is established if it is considered probable that a loss has been incurred. The accrual of interest on loans receivable is discontinued if the loan is 90 days past due based on the contractual terms of the loan unless the loan is adequately secured and in process of collection. In general, loans are placed on non-accrual status or charged off at an earlier date if collection of principal or interest is considered doubtful. Interest on these loans is accounted for on a cash basis until qualifying for return to accrual status. Loans are returned to accrual status when all principal and interest amounts contractually due are brought current and future payments are reasonably assured. Fair value is generally determined based on quoted market prices. For publicly traded securities for which quoted prices are unavailable, the Company determines fair value based on independent broker quotations and other observable market data. For securities traded only in private negotiations, the Company determines fair value based primarily on the cost of such securities, which is adjusted to reflect prices of recent placements of securities of the same issuer, financial data, projections and business developments of the issuer and other relevant information. Realized gains or losses represent the difference between the cost of securities sold and the proceeds realized upon sale and are recorded at the trade date. The Company uses primarily the first-in, first-out method to determine the cost of securities sold. The cost of securities is adjusted where appropriate to include a provision for a decline in value which is considered to be other than temporary. An other-than-temporary decline is considered to occur in investments where there has been a sustained reduction in fair value and where the Company does not expect to recover the cost basis of the investment prior to the time of sale or maturity. Since equity securities do not have a contractual cash flow or a maturity, the Company considers whether the price of an equity security is expected to recover within a reasonable period of time. For fixed maturity securities that the Company intends to sell or, more likely than not, would be required to sell, a decline in value below amortized cost is considered to be an other-than-temporary impairment (“OTTI”). The amount of OTTI is equal to the difference between amortized cost and fair value at the balance sheet date. For fixed maturity securities that the Company does not intend to sell or believes that it is more likely than not it would not be required to sell, a decline in value below amortized cost is considered to be an OTTI if the Company does not expect to recover the entire amortized cost basis of a security (i.e., the present value of cash flows expected to be collected is less than the amortized cost basis of the security). The portion of the decline in value considered to be a credit loss (i.e., the difference between the present value of cash flows expected to be collected and the amortized cost basis of the security) is recognized in earnings. The portion of the decline in value not considered to be a credit loss (i.e., the difference in the present value of cash flows expected to be collected and the fair value of the security) is recognized in other comprehensive income. Impairment assessments for structured securities, including mortgage-backed securities and asset-backed securities, collateralized debt obligations and corporate debt, are generally evaluated based on the performance of the underlying collateral under various economic and default scenarios that may involve subjective judgments and estimates by management. Modeling these securities involves various factors, such as projected default rates, the nature and realizable value of the collateral, if any, the ability of the issuer to make scheduled payments, historical performance and other relevant economic and performance factors. If an OTTI determination is made, a discounted cash flow analysis is used to ascertain the amount of the credit impairment. Real estate held for investment purposes is initially recorded at the purchase price, which is generally fair value, and is subsequently reported at cost less accumulated depreciation. Real estate taxes, interest and other costs incurred during development and construction are capitalized. Buildings are depreciated on a straight-line basis over the estimated useful lives of the building. Minimum rental income is recognized on a straight-line basis over the lease term. Income and expenses from real estate are reported as net investment income. The carrying value of real estate is reviewed for impairment and an impairment loss is recognized if the estimated undiscounted cash flows from the use and disposition of the property are less than the carrying value of the property.
Per share data	Per share data The Company presents both basic and diluted net income per share (“EPS”) amounts. Basic EPS is calculated by dividing net income by weighted average number of common shares outstanding during the year. Diluted EPS is based upon the weighted average number of common and common equivalent shares outstanding during the year and is calculated using the treasury stock method for stock incentive plans. Common equivalent shares are excluded from the computation in periods in which they have an anti-dilutive effect. Stock options for which the exercise price exceeds the average market price over the period have an anti-dilutive effect on EPS and, accordingly, are excluded from the calculation.
Deferred policy acquisition costs	Deferred policy acquisition costs Acquisition costs incurred in writing insurance and reinsurance business (primarily commissions and premium taxes) are deferred and amortized ratably over the terms of the related contracts. Ceding commissions received on reinsurance contracts are netted against acquisition costs and are recognized ratably over the life of the contract. Deferred policy acquisition costs are presented net of unearned ceding commissions. Deferred policy acquisition costs are reviewed to determine if they are recoverable from future income and, if not, are charged to expense. The recoverability of deferred policy acquisition costs is evaluated separately by each of our operating companies for each of their major lines of business. Future investment income is taken into account in measuring the recoverability of deferred policy acquisition costs.
Reserves for losses and loss expenses	Reserves for losses and loss expenses Reserves for losses and loss expenses are an accumulation of amounts determined on the basis of (1) evaluation of claims for business written directly by the Company; (2) estimates received from other companies for reinsurance assumed by the Company; and (3) estimates for losses incurred but not reported (based on Company and industry experience). These estimates are periodically reviewed and, as experience develops and new information becomes known, the reserves are adjusted as necessary. Such adjustments are reflected in the statements of income in the period in which they are determined. The Company discounts its reserves for excess and assumed workers' compensation claims using a risk-free or statutory rate.
Reinsurance ceded	Reinsurance ceded The unearned portion of premiums ceded to reinsurers is reported as prepaid reinsurance premiums and earned ratably over the policy term. The estimated amounts of reinsurance recoverable on unpaid losses are reported as due from reinsurers. To the extent any reinsurer does not meet its obligations under reinsurance agreements, the Company must discharge its liability. Amounts due from reinsurers are reflected net of funds held where the right of offset is present. The Company has provided reserves for estimated uncollectible reinsurance.
Deposit accounting	Deposit accounting Contracts that do not meet the risk transfer requirements of GAAP are accounted for using the deposit accounting method. Under this method, an asset or liability is recognized at the inception of the contract based on consideration paid or received. The amount of the deposit asset or liability is adjusted at subsequent reporting dates using the interest method with a corresponding credit or charge to interest income or expense.
Federal and foreign income taxes	Federal and foreign income taxes The Company files a consolidated income tax return in the U.S. and foreign tax returns in countries where it has overseas operations. The Company's method of accounting for income taxes is the asset and liability method. Under this method, deferred tax assets and liabilities are measured using tax rates currently in effect or expected to apply in the years in which those temporary differences are expected to reverse. Interest and penalties, if any, are reported as income tax expense. The Company believes there are no tax positions that would require disclosure under GAAP. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that all or a portion of the deferred tax assets will not be realized.
Foreign currency	Foreign currency Gains and losses resulting from foreign currency transactions (transactions denominated in a currency other than the entity's functional currency) are reported on the statements of income as other operating costs and expenses. Unrealized gains or losses resulting from translating the results of non-U.S. dollar denominated operations are reported in accumulated other comprehensive income. Revenues and expenses denominated in currencies other than U.S. dollars are translated at the weighted average exchange rate during the year. Assets and liabilities are translated at the rate of exchange in effect at the balance sheet date.
Property, furniture and equipment	Property, furniture and equipment Property, furniture and equipment are carried at cost less accumulated depreciation. Depreciation is calculated using the estimated useful lives of the respective assets.
Comprehensive income	Comprehensive income Comprehensive income encompasses all changes in stockholders' equity (except those arising from transactions with stockholders) and includes net income, net unrealized holding gains or losses on available for sale securities, unrealized foreign currency translation adjustments and changes in unrecognized pension obligations.
Goodwill and other intangible assets	Goodwill and other intangible assets Goodwill and other intangibles assets are tested for impairment on an annual basis and at interim periods where circumstances require. The Company's impairment test as of December 31, 2011 indicated that there were minimal impairment losses related to goodwill and other intangible assets. Intangible assets of $29 million are included in other assets as of December 31, 2011 and 2010.
Stock options	Stock options The costs resulting from all share-based payment transactions with employees are recognized in the consolidated financial statements using a fair-value-based measurement method.
Recent accounting pronouncements	Recent accounting pronouncements In October 2010, the FASB (Financial Accounting Standards Board) issued guidance regarding the treatment of costs associated with acquiring or renewing insurance contracts. This guidance modifies the definition of the types of costs that can be capitalized and specifies that the costs must be directly related to the successful acquisition of a new or renewed insurance contract. This guidance is effective for the quarter ending March 31, 2012. The Company plans to apply this guidance retrospectively and estimates that the effect of adopting this guidance will be a reduction in common stockholders' equity of between $50 million and $60 million as of January 1, 2012. All other recently issued but not yet effective accounting and reporting guidance is either not applicable to the Company, or is not expected to have a material impact on the Company.