Funds held - directly managed (cost: 2012, $1,230,199; 2011, $1,241,222) Funds Held Directly Managed Carrying value of segregated investment portfolio underlying the funds held - directly managed account comprised of fixed maturities, short-term investments, cash and cash equivalents, other assets and liabilities and accrued investment income. Substantially all of the fixed maturities, short-term investments and certain other invested assets in the segregated portfolio underlying the funds held-directly managed account are carried at fair value and accordingly all changes in the fair value are recorded in net realized and unrealized investment gains and losses in the Consolidated Statement of Operations. Net tax assets The aggregate tax effects of tax-paying jurisdictions in a net asset position as of the balance sheet date of all future tax deductions arising from temporary differences between tax basis and generally accepted accounting principles basis recognition of assets, liabilities, revenues and expenses, which can only be deducted for tax purposes when permitted under enacted tax laws; net of deducting the allocated valuation allowance, if any, to reduce such amount to net realizable value. Current tax receivables balances are also included in this amount. Net Tax Assets Net tax liabilities The cumulative amount of tax-paying jurisdictions in a net liability position as of the balance sheet date arising from temporary differences between accounting income in accordance with generally accepted accounting principles and tax-basis income that will result in future taxable income exceeding future accounting income. Includes current taxes payable. Net Tax Liabilities Net tax liabilities Other accumulated comprehensive loss (net of tax of: 2012, $6,780; 2011, $6,590) Other Accumulated Comprehensive Income Loss Net Of Tax This element represents Other Accumulated Comprehensive Income (Loss), Net of Tax. Includes cumulative gains (losses) on qualifying hedges and minimum pension liability, but excludes currency translation adjustment. Other accumulated comprehensive income (loss), net of tax Increase in unearned premiums Change during the period in the unearned gross premiums written (direct and assumed) and ceded premiums written. Change In Unearned Premiums (Increase) decrease in unearned premiums Change in other accumulated comprehensive loss, net of tax Change In Other Accumulated Comprehensive Income Loss Net Of Tax This element represents the change in other comprehensive income (loss), net of tax, for the period attributable to the parent entity. Includes gains (losses) on qualifying hedges, minimum pension liability, other comprehensive income on equity investments but excludes currency translation adjustment. Change in other accumulated comprehensive loss Reinsurance balances, net Increase Decrease in Reinsurance Balances Net The net change during the reporting period in the amounts due to or from other insurance companies when the reporting entity has assumed the risk on these policies. Includes changes in balances receivable and payable based on reports received from ceding companies, supplemented by the Company's estimates of premiums receivable and payable and other net receivable reinsurance balances. Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable Increase Decrease in Reinsurance Recoverable On Paid And Unpaid Losses Net Of Ceded Premiums Payable The net change during the reporting period in reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable. Funds held by reinsured companies and funds held - directly managed Increase Decrease in Funds Held By Reinsured Companies And Funds Held Directly Managed The net change during the reporting period in the balances of both Funds held by reinsured companies and Funds held - directly managed. Net tax assets and liabilities Increase Decrease in Net Tax Assets And Liabilities The net change during the reporting period of all current and deferred tax assets and liabilities. Unpaid losses and loss expenses including life policy benefits Increase Decrease in Unpaid Losses And Loss Expenses Including Life Policy Benefits The net change during the reporting period of unpaid losses and loss expenses and policy benefits for life and annuity contracts. Document And Entity Information [Abstract] Document And Entity Information Unaudited Condensed Consolidated Balance Sheets Parenthetical Disclosure Balance Sheet Parenthetical Disclosure [Abstract] Balance Sheet Parenthetical Disclosure Assets [Abstract] Balance Sheet Parenthetical Disclosure Assets Short-term investments, trading securities, amortized cost Short Term Investments Trading Securities Amortized Cost Short-term investments, trading securities, amortized cost Equities, trading securities, cost Equities Trading Securities Cost Equities, trading securities, cost Funds held - directly managed, cost Funds Held Directly Managed Cost Cost for funds held - directly managed Fixed maturities, trading securities, amortized cost Fixed Maturities Trading Securities Amortized Cost Fixed maturities, trading securities, amortized cost Balance Sheet Parenthetical Disclosures Equity [Abstract] Balance Sheet Parenthetical Disclosures Equity Disclosure - Organization [Abstract] Disclosure - Significant Accounting Policies [Abstract] Disclosure - Fair Value [Abstract] Life [Member] The Life segment includes mortality, longevity and health lines of business. Corporate And Other [Member] Corporate and Other is comprised of the capital markets and investment related activities of the Company, including principal finance transactions, insurance-linked securities and strategic investments, and its corporate activities, including other operating expenses. Non Life [Member] The Non-Life segment is the aggregation of the Non-Life subsegments Non-life subsegments [Axis] Non-life subsegments [Domain] The subsegments of the Non-life segment North America [Member] This sub-segment includes property, casualty, motor, multi-line, agriculture, surety and other risks generally originating in the United States. Global (Non-US) P&C [Member] This sub-segment includes casualty, property, and motor business generally originating outside of the United States. Global (Non-US) Specialty [Member] This sub-segment is comprised of business that is generally considered to be specialized due to the sophisticated technical underwriting required to analyze risks, and is global in nature. This sub-segment consists of several lines of business for which the Company believes it has developed specialized knowledge and underwriting capabilities. These lines of business include agriculture, aviation/space, credit/surety, energy, engineering, marine, specialty casualty, specialty property and other lines. Catastrophe Segment [Member] This sub-segment is comprised of the Company's catastrophe line of business. Underwriting result Underwriting Result Underwriting result is defined as the technical result net of other operating income and expenses. Underwriting result Change In Unrealized Gain On Financia lInstruments Table [Text Block] A table that provides a breakdown of the change in the unrealized gain/loss of financial instruments subject to the fair value option, by major category of investment. Change in fair value of financial instruments subject to fair value option Major Types OF Fixed Maturities [Axis] Fixed maturities broken down by major category. Major Types Of Fixed Maturities Domain Provides general categories of investments for which information about Fixed Maturities is required or determined to be disclosed. Gains and losses on trading activities (including both derivative and nonderivative instruments) recognized in the statement of financial performance, separately by major types of items (such as fixed income/interest rates, foreign exchange, equity, commodity, and credit). Major Types Of Fixed Maturities [Domain] Major Types Of Fixed Maturities [Domain] Equity Securities by Industry [Axis] This element provides details of investments in equities by industry. Equity Securities By Industry [Domain] This element provides details of investments in equities by industry. Consumer noncyclical [Member] This item represents investments by the reporting entity in equity securities issued by companies engaged in fishing and farming operations; the processing and production of food, beverages and tobacco; manufacturers of household and personal products; and providers of personal services. Technology [Member] This item represents investments by the reporting entity in equity securities issued by technology companies. Finance [Member] This item represents investments by the reporting entity in equity securities issued by banks, trusts and other finance companies. Communications [Member] This item represents investments by the reporting entity in equity securities issued by communications companies. Energy [Member] This item represents investments by the reporting entity in equity securities issued by energy companies. Industrials [Member] This item represents investments by the reporting entity in equity securities issued by industrial companies. Insurance [Member] This item represents investments by the reporting entity in equity securities issued by insurance companies. Other equities [Member] This item represents investments by the reporting entity in all other equity sectors. Mutual funds and exchange traded funds [Member] This item represents investments by the reporting entity in public exchange traded funds and mutual funds. Other Invested Assets By Type [Axis] This element provides details of other invested assets by type. Other Invested Assets By Type [Domain] This element provides details of other invested assets by type. Insurance-linked securities [Member] Various weather derivatives, weather futures and longevity total return swaps for which the underlying risks include parametric weather risks for the weather derivatives and weather futures, and longevity risk for the longevity total return swaps. Other invested assets other assets [Member ] This element provides details of other invested assets, excluding derivatives. Funds Held Directly Managed Investments Carried At Fair Value Funds held - directly managed Carrying value of those investments held in the segregated investment portfolio underlying the funds held - directly managed account that are carried at fair value, comprised of fixed maturities, short-term investments and certain other invested assets. All changes in the fair value are recorded in net realized and unrealized investment gains and losses in the Consolidated Statement of Operations. Allocated underwriting result Allocated Underwriting Result This element is defined as net premiums earned, other income or loss and allocated net investment income less life policy benefits, acquisition costs and other operating expenses for Life segment. Forward Contracts Net Investment Hedge [Member] Foreign exchange forward contracts are used to hedge a portion of the net investment exposure resulting from the translation of foreign subsidiaries and branches whose functional currency is other than the U.S. dollar. Net realized and unrealized investment gains and losses [Member] Net realized and unrealized investment gains and losses. Reinsurance Ratios [Abstract] Loss ratio Loss Ratio Percentage The loss ratio is obtained by dividing losses and loss expenses by net premiums earned. Acquisition ratio Acquisition Ratio Percentage The acquisition ratio is obtained by dividing acquisition costs by net premiums earned. Technical ratio Technical Ratio Percentage Technical ratio is defined as the sum of the loss ratio and the acquisition ratio. Other operating expense ratio Other Operating Expense Ratio Percentage Other operating expense ratio is obtained by dividing other operating expenses by net premiums earned Combined ratio Combined Ratio Percentage The combined ratio is defined as the sum of the technical ratio and the other operating expense ratio. Segment Geographic Distribution Table [Text Block] This element is used to illustrate the distribution of gross premiums written by geographic area for each of the years presented in the statement of earnings. Segment geographic distribution of premiums table Funds Held - Directly Managed investment [Member] Fair Value - Other Details [Line Items] Investments Not Carried At Fair Value Investments that the Company did not measure at fair value on the balance sheet date, which primarily relate to the company's investments that are accounted for using the cost method of accounting, equity method of accounting or investment company accounting. Aggregate carrying amount of other invested assets not carried at fair value Other Fair Value Disclosures [Table] Fair value disclosures of items not disclosed elsewhere Trading Securities By Balance Sheet Disclosure [Axis] This element represents the major balance sheet classifications of trading securities, whether shown as individual line items or combined into a separate account. Trading Securities By Balance Sheet Disclosure [Domain] Provides general categories of balance sheet classification of trading securities, either as separate line items or shown as a net number in a segregated account. Funds held directly managed [Member] This member shows the fair value and related disclosures of the portfolio underlying the funds held- directly managed account, which is composed of various types of investments, including fixed maturities, short-term investments and certain other invested assets. Other Net Assets Other net assets Other invested assets and liabilities carried at cost, which are related to the underlying business related to the funds held - directly managed account. Trading securities [Member] Investments which are shown on the face of the balance sheet, which do not form part of the portfolio underlying the funds held - directly managed account. Non US Sovereign Government Supranational And Government Related Debt Securities [Member] This category includes information about debt securities (such as bonds) issued by a national, local, or municipal government not included in the US Treasury and Government category. Fixed maturities and short term investments [Member] Fixed maturities and short term investments. Consumer cyclical [Member] This item represents investments by the reporting entity in equity securities issued by companies engaged in cyclical consumer goods. " Other invested assets Other Invested Assets Carried At Fair Value Other invested assets carried at fair value. Other invested assets [Member] Investments which are not otherwise included in another category. Other invested assets funds held directly managed [Member] Investments which are not otherwise included in another category that are included in the portfolio underlying the funds held - directly managed account. Losses and loss expenses and life policy benefits Losses And Loss Expenses And Life Policy Benefits Segment Disclosure Provision for benefits, claims and claims settlement expenses incurred during the period net of the effects of contracts assumed and ceded, by segment and subsegment. Cash and cash equivalents Represents the amount of cash and cash equivalents held in the portfolio underlying the funds held - directly managed account. Cash And Cash Equivalents Funds Held Directly Managed Accrued investment income Interest, dividends, rents, ancillary and other revenues earned but not yet received on investments held in the portfolio underlying the funds held - directly managed account. Accrued Investment Income Funds Held Directly Managed Segment disclosure for major brokers Segment Disclosures Summary Of Gross Written Premiums Attributable To Major Brokers Table [Text Block] Provides a summary of the percentage of gross premiums written through brokers accounting for more than 10% of the total. Aggregate liquidation value Preference Shares Aggregate Liquidation Value In the event of liquidation of the Company, the holders of outstanding preferred shares would have preference over the common shareholders and would receive a distribution of $25.00 per share, plus accrued and unpaid dividends. Preferred Stock Issued During Period Value New Issues Equity impact of the value of new preferred stock issued during the period. Issuance of preferred shares