| **Investments (Tables)**                                                                                                            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| **Investments [Abstract]**                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Analysis Of Cost Or Amortized Cost, Gross Unrealized Gains And Losses And Fair Value for Investments                                | The following table provides cost or amortized cost, gross unrealized gains, gross unrealized losses and fair value for our investment portfolio:  (Dollars in millions) Cost or amortized cost         Gross unrealized Fair valueAt June 30, 2015  gains losses Fixed maturity securities:        Corporate  $5,243 $350 $16 $5,577States, municipalities and political subdivisions 3,426 136 15 3,547Commercial mortgage-backed  278 7 2 283Government-sponsored enterprises 253 — 9 244Foreign government 10 — — 10Convertibles and bonds with warrants attached 7 — — 7United States government 7 — — 7Subtotal 9,224 493 42 9,675Equity securities:        Common equities 2,628 1,882 8 4,502Nonredeemable preferred equities 200 33 1 232Subtotal 2,828 1,915 9 4,734Total $12,052 $2,408 $51 $14,409At December 31, 2014        Fixed maturity securities:        Corporate  $5,117 $420 $11 $5,526States, municipalities and political subdivisions 3,267 178 2 3,443Commercial mortgage-backed  250 9 — 259Government-sponsored enterprises 213 — 5 208Foreign government 10 — — 10Convertibles and bonds with warrants attached 7 — — 7United States government 7 — — 7Subtotal 8,871 607 18 9,460Equity securities:        Common equities 2,583 2,099 3 4,679Nonredeemable preferred equities 145 35 1 179Subtotal 2,728 2,134 4 4,858Total $11,599 $2,741 $22 $14,318                                                                                                                                                        |
| Fair Values And Unrealized Losses by Investment Category And By The Duration Of The Securities' Continuous Unrealized Loss Position | The table below provides fair values and gross unrealized losses by investment category and by the duration of the securities’ continuous unrealized loss positions:(Dollars in millions) Less than 12 months 12 months or more Total  Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized lossesAt June 30, 2015      Fixed maturity securities:            Corporate  $715 $14 $65 $2 $780 $16States, municipalities and political subdivisions 6931456174915Commercial mortgage-backed  74 2 2 — 76 2Government-sponsored enterprises 90 2 152 7 242 9Foreign government 10 — — — 10 —Subtotal 1,582 32 275 10 1,857 42Equity securities:            Common equities 165 8 — — 165 8Nonredeemable preferred equities 66 1 5 — 71 1Subtotal 231 9 5 — 236 9Total $1,813 $41 $280 $10 $2,093 $51At December 31, 2014            Fixed maturity securities:            Corporate  $261 $8 $90 $3 $351 $11States, municipalities and political subdivisions 17 — 135 2 152 2Commercial mortgage-backed  3 — 23 — 26 —Government-sponsored enterprises 11 — 181 5 192 5Subtotal 292 8 429 10 721 18Equity securities:            Common equities 85 3 — — 85 3Nonredeemable preferred equities 16 — 17 1 33 1Subtotal 101 3 17 1 118 4Total $393 $11 $446 $11 $839 $22              |
| Investment Income, Realized Investment Gains And Losses And Change In Unrealized Investment Gains And Losses                        | The following table provides investment income, realized investment gains and losses, the change in unrealized investment gains and losses, and other items:(Dollars in millions)Three months ended June 30, Six months ended June 30, 2015 2014 2015 2014Investment income:       Interest$106 $103 $211 $207Dividends35 34 71 66Other 1 1 1 2Total142 138 283 275Less investment expenses2 2 4 4Total$140 $136 $279 $271        Realized investment gains and losses summary:       Fixed maturities:       Gross realized gains$7 $4 $10 $6Gross realized losses— (3) — (4)Other-than-temporary impairments(3) — (3) —Equity securities:       Gross realized gains56 16 100 34Gross realized losses— — (1) —Other-than-temporary impairments(1) — (1) (1)Other1 (3) 2 1Total$60 $14 $107 $36        Change in unrealized investment gains and losses:       Fixed maturities$(184) $93 $(138) $181Equity securities(135) 142 (224) 171Less income taxes112 (82) 127 (123)Total$(207) $153 $(235) $229                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |

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| - Definition
Tabular disclosure of investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums; proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales; components of comprehensive income.
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| - Definition
Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.
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| - Definition
For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value).
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