Short-term investments, trading securities, at fair value (amortized cost: 2011, $81,094; 2010, $49,132) This item represents short-term investments that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for short-term investments formerly categorized as available-for-sale or held-to-maturity which the entity held as of the date it opted to account for such securities at fair value. Such financial instruments that are held as of the reporting date are measured at fair value with unrealized holding gains and losses (the difference between fair value and the previously reported carrying amount) included in earnings Short-term investments, trading securities Short Term Investments Trading Securities At Fair Value Funds held - directly managed (cost: 2011, $1,506,386; 2010, $1,751,276) 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 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: 2011, $5,395; 2010, $4,872) 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 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 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. Reinsurance balances, net 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 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 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 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 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 Funds held - directly managed, cost 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 Other accumulated comprehensive loss, tax Other Accumulated Comprehensive Income Loss Tax Tax effect on Other Accumulated Comprehensive Income (Loss), including cumulative gains (losses) on qualifying hedges and minimum pension liability, but excluding currency translation adjustment. Sales of fixed maturities Sales Of Fixed Maturities Includes sales of fixed maturities designated as trading under the fair value option or fixed maturities categorized as available-for-sale or held-to-maturity which the Company held as of the date it opted to account for such securities at fair value. Redemptions of fixed maturities Redemptions Of Fixed Maturities Includes redemptions of fixed maturities designated as trading under the fair value option or fixed maturities categorized as available-for-sale or held-to-maturity which the Company held as of the date it opted to account for such securities at fair value. Purchases of fixed maturities Purchases Of Fixed Maturities Includes purchases of fixed maturities designated as trading under the fair value option or fixed maturities categorized as available-for-sale or held-to-maturity which the Company held as of the date it opted to account for such securities at fair value. Sales of equities Sales Of Equities Includes sales of equities designated as trading under the fair value option or equities categorized as available-for-sale or held-to-maturity which the Company held as of the date it opted to account for such securities at fair value. Purchases of equities Purchases Of Equities Includes purchases of equities designated as trading under the fair value option or equities categorized as available-for-sale or held-to-maturity which the Company held as of the date it opted to account for such securities at fair value. Disclosure - Organization [Abstract] Disclosure - Significant Accounting Policies [Abstract] Disclosure - Fair Value [Abstract] Disclosure - Derivatives [Abstract] Disclosure- Commitments and Contingencies [Abstract] Disclosure - Segment Reporting [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. Segment Reporting [Line Items] 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 (Loss) On Financial Instruments 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 Fair Value By Fair Value Hierarchy Level [Axis] Fair value information by level within the fair value hierarchy. Fair Value Measurements Fair Value Hierarchy [Domain] Provides the categories used to prioritize the inputs to valuation techniques used to measure fair value into levels. The fair value hierarchy gives the highest priority to quoted prices (unadjusted) in active markets for identical assets or liabilities (Level 1), inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly (Level 2), and the lowest priority to unobservable inputs (Level 3). Where the quoted price in an active market for the identical liability is not available, the Level 1 input is the quoted price of an identical liability when traded as an asset, or the quoted price for similar liabilities or similar liabilities when traded as an asset. 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 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. Credit default swaps (assumed risks) [Member] Credit default swaps used to replicate investment positions, generally with terms of five years or less and no recourse provisions associated. The counterparties are all highly rated financial institutions. 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 [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. Schedule Of Derivative Instruments Gain Loss In Statement Of Operations Table Text Block A table that identifies and provides pertinent information about a derivative or group of derivatives on a disaggregated basis and describes the location and amount of gains and losses reported in the statement of operations. Information that identifies a derivative or a group of derivatives by hedging designation. Hedging Designation [Axis] Hedging Designation Domain The name that identifies a derivative or a group of derivatives, by hedging designation. Designated as hedging instrument [Member] Derivative instruments designated as hedging instruments. 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. Basis Of Presentation And Significant Accounting Policies [Text Block] This element aggregates basis of presentation, or accounting, and significant accounting policies into a single text block. 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. Schedule Of Net Income Per Share Table [Text Block] Tabular disclosure of an entity's basic and diluted earnings per share calculations, including the disclosure of the numerators and denominators used in the calculations, including the effect that has been given to preferred dividends. Numerator [Abstract] Denominator [Abstract] Funds Held - Directly Managed investment [Member] Adjustments For New Accounting Pronouncements [Axis] Information by new accounting pronouncement. Type Of Adoption [Domain] Adoption and early adoption of sources of change to generally accepted accounting principles (GAAP) in the United States. The FASB released the FASB Accounting Standards Codification as the authoritative source of literature effective for interim and annual periods ending after September 15, 2009. As part of the Codification process, the FASB issues Accounting Standards Updates to amend the Codification but otherwise the Accounting Standards Updates are not authoritative in their own right. All previous accounting standards (such as FASB Statements of Financial Accounting Standards, FASB Interpretations, FASB Staff Positions, Emerging Issues Task Force Consensuses, other pronouncements of the FASB or other designated bodies, or other forms of GAAP are considered accounting pronouncements) were superseded upon the adoption of the Codification. For an interim period, references to the superseded standards are included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. FAS 159 [Member] Financial Accounting Standard 159: The Fair Value Option for Financial Assets and Financial Liabilities, Including an amendment of FASB Statement No. 115. Fair Value By Asset Class [Axis] Fair value information by class of asset. Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation By Asset Class [Domain] Represents classes of assets measured and disclosed at fair value. 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 Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Transfers Out Of Level 3 Transfers out of Level 3 Transfers out of assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period. Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Transfers Into Level 3 Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period. Transfers into Level 3 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. Cash acquired related to the acquisition of Paris Re Cash Acquired In Business Combination Cash balances acquired in a business combination. Share capital repayment paid to former shareholders of Paris Re Business Acquisition Capital Repayment Share capital repayment to the former shareholders of acquired company in a business combination. Business Combination Stock Issued During Period Value Treasury Stock Reissued Reissuance of treasury shares related to the acquisition of Paris Re Equity impact of the value of treasury stock (units) reissued during the period as part of a business combination. Upon reissuance of shares (units) from treasury, either the common or preferred stock (unit) reissued is outstanding. 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. Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Level 3 Assets Acquired Level 3 assets acquired from Paris Re Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy due to acquisitions during the period. Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Table] Schedule detailing credit quality information related to financing receivables by credit quality indicator and by class of financing receivable. Financing Receivable Information By Credit Quality Indicator [Axis] Information by credit quality indicator. Financing Receivable By Credit Quality Indicator [Domain] Credit quality indicators related to financing receivables. Performing [Member] Financing receivables assigned the grade of "Performing" Nonperforming [Member] Financing receivables assigned the grade of "Nonperforming" Financing Receivable Recorded Investment By Class Of Financing Receivable [Axis] Recorded investment in financing receivables by class of financing receivables. Financing Receivable Recorded Investment Class Of Financing Receivable [Domain] Listing of the classes of financing receivables. Classes of financing receivables generally are a disaggregation of a portfolio segment. Commercial Real Estate Other Receivable [Member] Class of financing receivables related to real estate financing receivables other than those related to commercial real estate construction. Residential Prime Financing Receivable [Member] Class of financing receivables related to prime residential financing receivables. Financing Receivable Recorded Investment [Line Items] Financing Receivable The amount of the recorded investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, accounts receivable (with terms exceeding one year), notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Financing receivable 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. Aggregate liquidation value, Series C 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, or an aggregate value of $290 million, plus accrued and unpaid dividends. Series C Preference Shares Aggregate Liquidation Value Aggregate liquidation value, Series D 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, or an aggregate value of $230 million, plus accrued and unpaid dividends. Series D Preference Shares Aggregate Liquidation Value 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 Non US sovereign government supranational and government related [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. Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Purchases Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. Purchases Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Sales Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. Sales Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Settlements Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. Settlements Share Options and Other [Member] Notes to Net Income Per Share [Abstract] Notes to Net (Loss) Income Per Share [Abstract] Weighted Average Number Of Common Shares And Common Share Equivalents Outstanding Including Potentially Dilutive Securities Weighted average number of common shares and common share equivalents outstanding including potentially dilutive securities The weighted average number of common and common share equivalents outstanding, including potentially dilutive securities which were not taken into account in the computation of diluted earnings per share because to do so would have been anti-dilutive. Amortization of net premium on investments Accretion Amortization Of Discounts And Premiums Investments Accrued investment income Accrued Investment Income Receivable Accumulated other comprehensive income: Accumulated Other Comprehensive Income Loss Net Of Tax Abstract Additional paid-in capital Additional Paid In Capital Balance Sheet Amortization of intangible assets Amortization of intangible assets Amortization Of Intangible Assets Stock Compensation Plans [Member] Mortgage/asset-backed securities [Member] Statement Of Financial Position Abstract Unaudited Condensed Consolidated Balance Sheets Condensed Balance Sheets - Parent Company Only Gross premiums written Assumed Basic net (loss) income Cash and cash equivalents, at fair value, which approximates amortized cost Cash and cash equivalents Cash and cash equivalents - beginning of period Cash and cash equivalents - end of period Interest paid Changes in: Increase Decrease In Operating Capital Abstract Unpaid losses and loss expenses Gross liability at beginning of year Gross liability at end of year Liability For Claims And Claims Adjustment Expense Commitments and Contingencies Commitments And Contingencies Disclosure Text Block Common shares, shares issued Common shares (par value $1.00, issued: 2011, 84,271,175 shares; 2010, 84,033,089 shares) Common Stock Value Comprehensive (loss) income Comprehensive Income Net Of Tax Concentration Risk By Type [Axis] Concentration Risk Type [Domain] Corporate [Member] Asset-backed securities [Member] Credit default swaps (protection purchased) [Member] Currency translation adjustment Accumulated Other Comprehensive Income Loss Foreign Currency Translation Adjustment Net Of Tax Deferred acquisition costs Deferred Policy Acquisition Costs Acquisition costs Deferred Policy Acquisition Cost Amortization Expense Acquisition costs Deferred acquisition costs Increase Decrease In Deferred Policy Acquisition Costs Deposit assets Deposit Contracts Assets Deposit liabilities Deposit Contracts Liabilities Derivative Name [Domain] Fair value Derivative By Nature [Axis] Derivatives, net [Member] Derivatives Derivative [Line Items] Diluted net (loss) income Cash dividends paid to shareholders Cash dividends paid to shareholders Payments Of Ordinary Dividends Purchase of capital efficient notes Early Repayment Of Subordinated Debt Interest in earnings of equity investments Income Loss From Equity Method Investments Effect of foreign exchange rate changes on cash Equity securities [Member] Losses and loss expenses and life policy benefits Net Policyholder Benefits And Claims Incurred Net Foreign currency option contracts [Member] Futures contracts [Member] Policy benefits for life and annuity contracts Policy benefits for life and annuity contracts Net realized and unrealized investment gains (losses) Net realized and unrealized investment (losses) gains Gain Loss On Investments Net realized and unrealized investment gains (losses) Net realized and unrealized investment (losses) gains Net realized gain on purchase of capital efficient notes Gains Losses On Extinguishment Of Debt Net realized gain on purchase of capital efficient notes Technical result Technical result Unaudited Condensed Consolidated Statements of Operations and Comprehensive Income Income Statement Abstract Intangible assets Intangible Assets Net Excluding Goodwill Goodwill Goodwill Interest expense Interest expense Interest Expense Interest rate swaps [Member] Net investment income Investment Income Net Net investment income (loss) Total investments Investments Proceeds from issuance of senior notes Debt related to capital efficient notes Junior Subordinated Notes Total liabilities Total liabilities Liabilities Liabilities Liabilities Abstract Total liabilities and shareholders' equity Total liabilities and shareholders' equity Cash flows from operating activities Net Cash Provided By Used In Operating Activities Continuing Operations Abstract Net cash (used in) provided by financing activities Net Cash Provided By Used In Financing Activities Net cash used in financing activities Cash flows from financing activities Net cash used in investing activities Net Cash Provided By Used In Investing Activities Net cash used in investing activities Cash flows from investing activities Net cash used in operating activities Net Cash Provided By Used In Operating Activities Net cash used in operating activities Net (loss) income Net Income Loss Net (loss) income Net (loss) income available to common shareholders Net Income Loss Available To Common Stockholders Basic Net (loss) income available to common shareholders (Decrease) increase in cash and cash equivalents Increase (decrease) in cash and cash equivalents Net premiums written Net Premiums Written Net New Accounting Pronouncement [Member] Impact of adopting fair value option for certain investments, net of tax Recent Accounting Pronouncements Description Of New Accounting Pronouncements Not Yet Adopted Total revenues Total revenues Revenues Revenues Revenues Abstract Other assets Other, net Other Assets Other net changes in operating assets and liabilities Increase Decrease In Other Operating Capital Net Other, net Other, net Other, net Payments For Proceeds From Other Investing Activities Other, net Contract fees on forward sale agreement Contract fees on forward sale agreement Preferred shares, shares issued Preferred shares, shares outstanding Preferred shares, par value Reinsurance balances receivable Premiums Receivable At Carrying Value Issuance of common shares Sales and redemptions of short-term investments Purchases of short-term investments Payments To Acquire Short Term Investments Reinsurance recoverable on paid and unpaid losses Reinsurance Recoverables Repayment of debt Repayments Of Long Term Debt Repurchase of common shares Payments for Repurchase of Common Stock Retained earnings Retained Earnings Accumulated Deficit Net premiums earned Net Net premiums earned Segment Information Segment reporting table Schedule Of Segment Reporting Information By Segment [Table] Series C cumulative preferred shares (par value $1.00, issued and outstanding: 2011 and 2010, 11,600,000 shares; aggregate liquidation value: 2011 and 2010, $290,000) Series D cumulative preferred shares (par value $1.00, issued and outstanding: 2011 and 2010, 9,200,000 shares; aggregate liquidation value: 2011 and 2010, $230,000) Short-term investments [Member] Unaudited Condensed Consolidated Statements of Cash Flows Statement Of Cash Flows Abstract Unaudited Condensed Consolidated Statements of Shareholders Equity Shareholders' Equity Stockholders Equity Abstract Supplemental cash flow information: Taxes paid Common shares held in treasury Reissuance of treasury shares, other Stock Issued During Period Value Treasury Stock Reissued US Government and agencies [Member] Current portion of long-term debt Unsecured Debt Current Use of Estimates Weighted average number of common shares and common share equivalents outstanding Weighted average number of common shares and common share equivalents outstanding Share options and other Weighted average number of common shares outstanding Weighted Average Number Of Shares Outstanding Basic Common shares [Member] Preferred shares Total assets Total assets Assets Fixed maturities, trading securities, at fair value (amortized cost: 2011, $12,968,674; 2010, $12,394,797) Fixed maturities, trading securities Equities, trading securities, at fair value (cost: 2011, $908,144; 2010, $942,745) Equities, trading securities Major Types Of Trading Securities And Assets [Domain] Concentration Risk By Benchmark [Axis] Concentration Risk Benchmark [Domain] Dividends declared per common share Common Stock Dividends Per Share Declared Fair Value By Balance Sheet Grouping [Table] Fair Value By Balance Sheet Grouping Disclosure Item Amounts [Axis] Fair Value Disclosure Item Amounts Domain Carrying value [Member] Fair value [Member] Estimate of Fair Value, Fair Value Disclosure [Member] Fair value of financial instrument liabilities [Line Items] Statement [Table] Antidilutive Securities Name Domain Total Investments Fair Value Disclosure Assets Assets Abstract Statement [Line Items] Financial instruments measured at fair value and categorized between Levels 1, 2 and 3 Fair Value Assets Measured On Recurring Basis Text Block Fair Value Assets Measured On Recurring Basis [Table] Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items] Reconciliation of beginning and ending balances for all financial instruments measured at fair value using Level 3 inputs Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Text Block Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Table] Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] Balance, beginning of period Balance, end of period Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Value Net purchases, sales and settlements Realized and unrealized investment gains (losses) included in net income Net transfers (out of)/ into Level 3 Change in unrealized investment gains (losses) relating to assets held at the end of period Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) Fair Value Option Quantitative Disclosures [Table] Fair Value Option Quantitative Disclosures By Eligible Item Or Group [Axis] Fair Value, Option, Quantitative Disclosures [Line Items] Change in unrealized gain/loss Fair Value Disclosures [Text Block] Carrying values and fair values of financial instrument liabilities Fair Value By Balance Sheet Grouping Text Block Debt related to senior notes Senior Notes Class Of Stock [Domain] Class Of Stock [Domain] Common shares held in treasury, at cost (2011, 16,831,534 shares; 2010, 14,046,895 shares) Treasury Stock Value Common shares held in treasury, at cost (2011, 16,831,534 shares; 2010, 14,046,895 shares) Fixed maturities [Member] Share options excluded from computation of weighted average number of common share and common share equivalents outstanding Antidilutive Securities Excluded From Computation Of Earnings Per Share By Antidilutive Securities [Axis] Antidilutive securities excluded from computation of earnings (loss) per share [Line Items] Per share data Disclosure - Net (Loss) Income Per Share [Abstract] Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table] Schedule Of Trading Securities And Other Trading Assets Major Types Of Trading Securities And Assets [Axis] (Loss) income before taxes and interest in earnings of equity investments (Loss) income before taxes and interest in earnings of equity investments Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments Statement Class Of Stock [Axis] Unearned premiums Unearned Premiums Other reinsurance balances payable Reinsurance Payable Common shares, par value Common shares held in treasury, shares Shares held in treasury Change in currency translation adjustment Other Comprehensive Income Foreign Currency Transaction And Translation Gain Loss Arising During Period Net Of Tax Funds held by reinsured companies Funds Held Under Reinsurance Agreements Asset Income tax (benefit) expense Total income tax expense (benefit) Income tax benefit (expense) Net foreign exchange gains Net foreign exchange gains Foreign Currency Transaction Gain Loss Before Tax Unearned premiums Increase Decrease In Unearned Premiums Cumulative preferred shares Preferred Stock Value Other operating expenses Other operating expenses Other Expenses Comprehensive (loss) income Comprehensive Income Net Of Tax Abstract Other income Other Income Expenses Benefits Losses And Expenses Abstract Total expenses Benefits Losses And Expenses Total expenses Statement Equity Components [Axis] Additional paid-in capital Retained earnings Accumulated other comprehensive income Equity Component [Domain] Issuance of common shares Stock Issued During Period Value New Issues Issuance of common shares related to the acquisition of Paris Re Statement Business Segments [Axis] Segment Business [Domain] Basis of Accounting Repurchase of common shares Cost of shares repurchased Treasury Stock Value Acquired Cost Method Dividends on common shares Dividends Common Stock Dividends on preferred shares Preferred dividends Dividends Preferred Stock Net (Loss) Income Per Share Earnings Per Share Text Block Total shareholders' equity Balance at beginning of period Balance at end of period Total shareholders' equity Stockholders Equity Including Portion Attributable To Noncontrolling Interest Other invested assets Other invested assets Other mortgage-backed securities [Member] Adjustments to reconcile net (loss) income to net cash provided by operating activities: Adjustments Noncash Items To Reconcile Net Income Loss To Cash Provided By Used In Operating Activities Abstract Organization Residential mortgage-backed securities [Member] Accounts payable, accrued expenses and other Accounts Payable And Accrued Liabilities Current And Noncurrent Total return swaps [Member] Foreign exchange forward contracts [Member] Interest rate derivatives [Member] Other [Member] Net foreign exchange gains and losses [Member] Derivates not designated as hedges [Member] Income Statement And Other Comprehensive Income Location Domain Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments Gain Loss By Income Statement Location [Axis] Derivative Instruments Gain Loss By Hedging Relationship By Income Statement Location By Derivative Instrument Risk [Table] Effective portion of net investment hedging derivatives Amount of gain(loss) on derivatives recognized in income Schedule Of Derivative Instruments In Statement Of Financial Position Fair Value Text Block Fair Values Derivatives Balance Sheet Location By Derivative Contract Type By Hedging Designation [Table] Amendment description Amendment flag Current fiscal year end date Document period end date Entity well known seasoned issuer Entity voluntary filers Entity current reporting status Entity filer category Entity public float Entity registrant name Entity central index key Entity common stock shares outstanding Document Fiscal Year Focus Document Fiscal Period Focus Document type Notional value