Document and Entity Information [Abstract] Document and Entity Information. Property and Equipment [Abstract] Property, Plant and Equipment Disclosure [Text Block] Property and Equipment Income Tax Disclosure [Text Block] Federal and Foreign Income Tax Shareholders' Equity [Abstract] Shareholders' Equity Stockholders' Equity Note Disclosure [Text Block] Shareholders' Equity Property and Equipment (Tables) [Abstract] Property and Equipment (Details) [Abstract] Property, Plant and Equipment, Net Property, Plant and Equipment, Net, Ending Balance Property, Plant and Equipment, Net, Beginning Balance Property and Equipment, Net of accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Beginning Balance Accumulated depreciation Accumulated depreciation Property, Plant and Equipment, Gross Property, Plant and Equipment, Gross, Ending Balance Property, Plant and Equipment, Gross, Beginning Balance Property, Plant and Equipment, Gross, Total Cost Shareholders' Equity (Details) [Abstract] Property And Equipment (Textuals) [Abstract] Property and Equipment (Textuals) [Abstract] Shares, Issued Balance, end of year Balance, beginning of year Common stock, outstanding, end of year Common Stock, Shares, Outstanding Common Stock, Shares, Outstanding, Beginning Balance Common stock outstanding, end of year Entity Registrant Name Entity Central Index Key Document Type Document Period End Date Amendment Flag Document Fiscal Year Focus Document Fiscal Period Focus Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Common Stock, Shares Outstanding (actual number of shares) Unpaid Losses and Loss Expenses (Details) [Abstract] Unpaid Losses and Loss Expenses (Textuals) [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Reconciliation of the beginning and ending liability for unpaid losses and loss expenses [Abstract] Reinsurance Recoverables Reinsurance Recoverable on Unpaid Losses and Loss Expenses Reinsurance recoverable, beginning of year Reinsurance Recoverable on Unpaid Losses and Loss Expenses Net Unpaid Losses and Loss Expenses Liability for Unpaid Claims and Claims Adjustment Expense, Net Net liability, beginning of year Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total Net incurred losses and loss expenses [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year Current year Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years Prior years Prior years Favorable development on net unpaid losses and loss expenses established as of the previous year end Net payments for losses and loss expenses [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Prior years Unpaid Losses and Loss Expenses (Tables) [Abstract] Reconciliation Of Beginning And Ending Liability For Unpaid Losses And Loss Expenses Text Block Reconciliation of the beginning and ending liability for unpaid losses and loss expenses Reconciliation of the beginning and ending liability for unpaid losses and loss expenses, net of reinsurance recoverable, and a reconciliation of the net liability to the corresponding liability on a gross basis. Favorable development experienced in the professional liability classes other than fidelity Favorable development experienced in the professional liability classes other than fidelity. Favorable development experienced in the professional liability classes other than fidelity Favorable development experienced in the professional liability classes other than fidelity Favorable development experienced in professional liability classes other than fidelity, outside domestic country Favorable development experienced in the professional liability classes other than fidelity, outside the United States Favorable development experienced in the professional liability classes other than fidelity, outside domestic country. Aggregate favorable development experienced in the commercial and personal liability classes Aggregate favorable development experienced in the commercial and personal liability classes. Aggregate favorable development experienced in the commercial and personal liability classes Aggregate favorable development experienced in the commercial and personal liability classes Favorable development experienced in the workers' compensation class Favorable development experienced in the workers' compensation class. Favorable development experienced in the workers' compensation class Property and Casualty Insurance Group [Member] Represents the property and casualty insurance subsidiaries of the Corporation. Corporate and Other [Member] Represents the non-insurance subsidiaries of the Corporation. Statutory Accounting Practices, Statutory Capital and Surplus, Balance Statutory Accounting Practices, Statutory Capital and Surplus, Balance, Total Policyholders' surplus Statutory Accounting Practices, Statutory Net Income Amount Statutory net income Segment Reporting Information, by Segment [Axis] Reporting Segment [Domain] Selected Quarterly Financial Information [Abstract] Quarterly Financial Data [Abstract] Net Income (Loss) Attributable to Parent Net Income NET INCOME GAAP net income (loss) Earnings Per Share, Basic Basic earnings per share Basic Earnings Per Share, Diluted Diluted earnings per share Diluted Underwriting ratios [Abstract] Losses to premiums earned Represents the ratio of losses and loss expenses to premiums earned less dividends to policyholders. Losses to premiums earned Expenses to premiums written Represents the ratio of statutory underwriting expenses to premiums written less dividends to policyholders. Expenses to premiums written Shareholder's Equity Textuals [Abstract] Federal and Foreign Income Tax (Tables) [Abstract] Federal and Foreign Income Tax (Details) [Abstract] Federal and Foreign Income Tax [Abstract] Income tax expense Current tax, Foreign Current Foreign Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Deferred tax (credit), principally United States Effective Income Tax Rate, Continuing Operations Effective Income Tax Rate, Continuing Operations, Total Actual tax, Percentage of Pre-Tax Income Total deferred income tax assets Deferred Tax Assets, Net Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits Postretirement benefits Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Unearned Premiums Reserve Unearned premiums Deferred Tax Assets (Liabilities), Net Net deferred income tax asset Deferred Tax Assets (Liabilities), Net, Total Deferred Income Tax Total deferred income tax liabilities Deferred Tax Liabilities Deferred Tax Liabilities, Total Other Invested Assets This item represents other invested assets which include investments such as private equity limited partnerships carried at the Corporation's equity in the net assets of the partnerships and warrants reported at fair value. Other invested assets Other Invested Assets Other invested assets Deferred Tax Liabilities, Undistributed Foreign Earnings Unremitted earnings of foreign subsidiaries Income Tax Expense (Benefit) Federal and Foreign Income Tax (Credit) Federal and Foreign Income Tax Actual tax Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate Tax at statutory federal income tax rate, Percentage of Pre-Tax Income Effective Income Tax Rate Reconciliation, Tax Exempt Income Tax exempt interest income, Percentage of Pre-Tax Income Unpaid Losses and Loss Expenses [Abstract] Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Unpaid Losses and Loss Expenses Depreciation Premises and Equipment Depreciation Depreciation expense related to property and equipment Income tax expense and taxes paid [Text Block] Income tax expense and taxes paid A table disclosing components of income tax expense and taxes paid. Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Cumulative Effects of Changes in Accounting Principles, Noncontrolling Interest Loss before Federal and Foreign Income Tax and Equity in Net Income of Consolidated Subsidiaries Income before federal and foreign income tax Total income before income tax Income before federal and foreign income tax INCOME BEFORE FEDERAL AND FOREIGN INCOME TAX Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate Tax at statutory federal income tax rate Income Tax Reconciliation, Tax Exempt Income Tax exempt interest income Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Loss Reserves Unpaid losses and loss expenses Deferred Tax Assets, Tax Credit Carryforwards, Foreign Foreign tax credits Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation Employee compensation Repurchase of shares Repurchase of shares Repurchase of shares Number of shares that have been repurchased and retired during the period and shares that have been repurchased during the period and are being held in treasury. Stock Issued During Period, Shares, Share-based Compensation Share activity under stock-based employee compensation plans Share activity under stock-based employee compensation plans Stockholders' Equity Attributable to Parent Balance, End of Year Balance, End of Year TOTAL SHAREHOLDERS' EQUITY Shareholders' equity Revenues Total Revenues TOTAL REVENUES Revenues Total Losses And Expenses TOTAL LOSSES AND EXPENSES Losses and expenses The total amount of losses and expenses recognized during the period. Maximum Dividend Distribution Maximum dividend distribution that may be made by the property and casualty insurance subsidiaries to parent company during 2011 without prior regulatory approval. Maximum dividend distribution that may be made by the domestic property and casualty subsidiaries to parent company during the given period without prior regulatory approval. Consolidated Summary of Investments - Other Than Investments In Related Parties [Abstract] Consolidated Summary Of Investments - Other Than Investments In Related Parties [Abstract] Summary of Investments, Other than Investments in Related Parties [Text Block] CONSOLIDATED SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES Condensed Financial Information of Parent Company [Abstract] Condensed Financial Information of Parent Company Only Disclosure [Text Block] CONDENSED FINANCIAL INFORMATION OF PARENT COMPANY Consolidated Supplementary Insurance Information [Abstract] Supplementary Insurance Information, for Insurance Companies Disclosure [Text Block] CONSOLIDATED SUPPLEMENTARY INSURANCE INFORMATION Quarterly Financial Data (Details) [Abstract] Comparison Of Shareholders Equity On Gaap Basis And Policyholders Surplus On Statutory Basis Text Block Comparison of shareholders' equity on a GAAP basis and policyholders' surplus on a statutory basis A table showing the comparison of shareholders' equity prepared based on generally accepted accounting principles and policyholders' surplus prepared on an accounting basis prescribed or permitted by insurance regulatory authorities. Comparison of shareholders' equity on a GAAP basis and policyholders' surplus on a statutory basis [Abstract] Comparison of shareholders' equity on a GAAP basis and policyholders' surplus on a statutory basis [Abstract] Comparison of GAAP and statutory net income (loss) [Abstract] Comparison of GAAP and statutory net income (loss) Comparison Of Gaap And Statutory Net Income Loss Text Block Comparison of GAAP and statutory net income (loss) A table showing the comparison of net income or loss prepared based on generally accepted accounting principles and prepared on an accounting basis prescribed or permitted by insurance regulatory authorities. Statement [Table] Statement, Equity Components [Axis] Equity Component [Domain] Parent [Member] Common Stock [Member] Treasury Stock, at Cost [Member] Statement [Line Items] QUARTERLY FINANCIAL DATA Quarterly Financial Information [Text Block] Quarterly Financial Data [Abstract] Deferred Policy Acquisition Costs [Abstract] Deferred Policy Acquisition Costs Deferred Policy Acquisition Costs [Text Block] Deferred Policy Acquisition Costs Deferred Policy Acquisition Costs And Related Amortization [Text Block] Deferred policy acquisition costs and related amortization A table that rolls forward the deferred policy acquisition costs balance identifying policy acquisition costs deferred and the related amortization reflected in operating results during the period. Debt and Credit Arrangements [Abstract] Debt and Credit Arrangements (Tables) [Abstract] Deferred Policy Acquisition Costs (Details) [Abstract] Deferred Policy Acquisition Cost, Amortization Expense Amortization of Deferred Policy Aquisition Costs Amortization during year Amortization during year Amortization of Deferred Policy Acquisition Costs Debt and Credit Arrangements (Details) [Abstract] Debt and Credit Arrangements (Textuals) [Abstract] Debt and Credit Arrangements (Textuals) [Abstract] 6% notes due November 15, 2011 [Member] Represents notes due on November 15, 2011. 5.2% notes due April 1, 2013 [Member] Represents notes due on April 1, 2013. 5.75% notes due May 15, 2018 [Member] Represents notes due on May 15, 2018. 6% notes due May 11, 2037 [Member] Represents notes due on May 11, 2037. 6.5% notes due May 15, 2038 [Member] Represents notes due on May 15, 2038. 6.6% debentures due August 15, 2018 [Member] Represents debentures due on August 15, 2018. 6.8% debentures due November 15, 2031 [Member] Represents debentures due on November 15, 2031. Long-term Debt Long term debt (Note-7) Long-term Debt, Total Long Term Debt Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Debt Instrument [Line Items] Employee Benefits (Tables) [Abstract] Pension and Other Postretirement Benefits Disclosure [Text Block] Employee Benefits Pension Benefits [Member] Other Postretirement Benefits [Member] Defined Benefit Plan, Funded Status of Plan [Abstract] Funded status of the pension and other postretirement benefit plans [Abstract] Employee Benefits (Details) [Abstract] Defined Benefit Plan, Benefit Obligation Benefit obligation, end of year Benefit obligation, beginning of year Benefit obligation Defined Benefit Plan, Fair Value of Plan Assets Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance Fair value of investments of other postretirement benefit plan assets in pooled funds Plan assets at fair value Plan assets at fair value Defined Benefit Plan, Funded Status of Plan Funded status at end of year, included in other liabilities Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] Weighted average assumptions used to determine the benefit obligations [Abstract] Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Discount rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Rate of compensation increase Weighted Average Assumptions Used To Determine Benefit Obligations Text Block Weighted average assumptions used to determine the benefit obligations A table disclosing the weighted average assumptions used to determine the benefit obligations. Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] Components of net pension and other postretirement benefit costs reflected in net income [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost Total benefit costs reflected in net income Defined Benefit Plan, Expected Return on Plan Assets Expected return on plan assets Expected return on plan assets Expected return on plan assets Defined Benefit Plan, Interest Cost Interest cost Interest cost Defined Benefit Plan, Service Cost Service cost Service cost Weighted Average Assumptions Used To Determine Net Pension And Other Postretirement Benefit Costs Text Block Weighted average assumptions used to determine net pension and other postretirement benefit costs A table disclosing the weighted average assumptions used to determine net pension and other postretirement benefit costs. Defined Benefit Plans Disclosures, Defined Benefit Plans [Axis] Defined Benefit Plans [Domain] Stock-Based Employee Compensation Plans (Tables) [Abstract] Additional Information With Respect To Restricted Stock Units And Performance Units Text Block Additional information with respect to restricted stock units and performance units A table that rolls forward the number of nonvested shares with respect to restricted stock units and performance units. The table also discloses weighted average fair value at grant date with respect to restricted stock units and performance units. Stock-Based Employee Compensation Plans (Details) [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Stock option pricing weighted average assumptions [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Risk-free interest rate Stock Based Employee Compensation Plans Textuals Abstract Stock-Based Employee Compensation Plans (Textuals) [Abstract] Total income tax benefit related to stock-based compensation plans Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Employee Service Share-based Compensation, Unrecognized Compensation Costs on Nonvested Awards Unrecognized compensation cost related to nonvested awards Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Shares available for grant Maximum Offering Period For Stock Purchase Plan Maximum offering period in months for the stock purchase plan The maximum offering period for the stock purchase plan during which employees are eligible to receive rights to purchase shares. Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options Tax benefit realized with respect to the exercise of stock options Total intrinsic value of stock options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value Number of Shares Exercised Cash received from the exercise of stock options Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract] Additional information with respect to stock options [Abstract] Stock Based Employee Compensation Plans [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Weighted Average Exercise Price on Number of Shares Outstanding, December 31, 2010 Weighted Average Exercise Price on Number of Shares Outstanding, January 1, 2010 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Number of Shares Outstanding, December 31, 2010 Number of Shares Outstanding, January 1, 2010 Weighted Average Exercise Price Granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Weighted average fair value per stock option granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period Number of Shares, Granted Number of Shares, Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Weighted Average Health Care Cost Trend Rate Assumptions Text Block Weighted average health care cost trend rate assumptions A table disclosing the weighted average health care cost assumptions used to measure the expected cost of medical benefits. Short term investments [Member] Equity securities [Member] Other assets [Member] Corporate bonds [Member] Mortgage-backed securities [Member] Fair Values Of Pension Plan Assets Text Block Fair values of the pension plan assets A table disclosing investments that have been segregated and restricted (usually in a trust) to provide benefits at their fair value as of the measurement date. Defined Benefit Plan, Estimated Future Benefit Payments [Abstract] Estimated benefits expected to be paid [Abstract]