| **Investments**            | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            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| **Investments [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 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| Investments                | 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 September 30, 2016  gains losses Fixed maturity securities:        Corporate  $5,621 $375 $14 $5,982States, municipalities and political subdivisions 3,619 222 1 3,840Commercial mortgage-backed  290 18 — 308Government-sponsored enterprises 108 — — 108Foreign government 10 — — 10Convertibles and bonds with warrants attached 5 — — 5United States government 4 — — 4Subtotal 9,657 615 15 10,257Equity securities:        Common equities 2,982 2,156 61 5,077Nonredeemable preferred equities 187 40 — 227Subtotal 3,169 2,196 61 5,304Total $12,826 $2,811 $76 $15,561At December 31, 2015        Fixed maturity securities:        Corporate  $5,294 $255 $96 $5,453States, municipalities and political subdivisions 3,440 172 1 3,611Commercial mortgage-backed  287 4 2 289Government-sponsored enterprises 284 — 6 278Foreign government 10 — — 10Convertibles and bonds with warrants attached 5 — — 5United States government 4 — — 4Subtotal 9,324 431 105 9,650Equity securities:        Common equities 2,749 1,787 51 4,485Nonredeemable preferred equities 189 32 — 221Subtotal 2,938 1,819 51 4,706Total $12,262 $2,250 $156 $14,356          The net unrealized investment gains in our fixed-maturity portfolio are primarily the result of the continued low interest rate environment that increased the fair value of our fixed-maturity portfolio. Our commercial mortgage-backed securities had an average rating of Aa1/AA at September 30, 2016, and December 31, 2015. The seven largest unrealized investment gains in our common stock portfolio are from Honeywell International Incorporated (NYSE:HON), Exxon Mobil Corporation (NYSE:XOM),  The Procter & Gamble Company (NYSE:PG), Johnson & Johnson (NYSE:JNJ), BlackRock Inc. (NYSE:BLK), 3M Co (NYSE:MMM) and Microsoft Corporation (Nasdaq:MSFT), which had a combined gross unrealized gain of $649 million. At September 30, 2016, Apple Inc. (Nasdaq:AAPL) was our largest single common stock holding with a fair value of $158 million, which was 3.1 percent of our publicly traded common stock portfolio and 1.0 percent of the total investment portfolio. 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 September 30, 2016      Fixed maturity securities:            Corporate  $166 $4 $200 $10 $366 $14States, municipalities and political subdivisions 128 1 — — 128 1Commercial mortgage-backed  8 — 3 — 11 —Government-sponsored enterprises 43 — — — 43 —Subtotal 345 5 203 10 548 15Equity securities:            Common equities 177 6 203 55 380 61Total $522 $11 $406 $65 $928 $76At December 31, 2015            Fixed maturity securities:            Corporate  $1,099 $63 $133 $33 $1,232 $96States, municipalities and political subdivisions 47 1 22 — 69 1Commercial mortgage-backed  103 2 2 — 105 2Government-sponsored enterprises 100 2 127 4 227 6Subtotal 1,349 68 284 37 1,633 105Equity securities:            Common equities 270 51 — — 270 51Nonredeemable preferred equities 35 — — — 35 —Subtotal 305 51 — — 305 51Total $1,654 $119 $284 $37 $1,938 $156             Contractual maturity dates for fixed-maturity investments were:(Dollars in millions) Amortizedcost Fairvalue % of fairvalueAt September 30, 2016   Maturity dates:      Due in one year or less $446 $452 4.4%Due after one year through five years 3,049 3,265 31.8Due after five years through ten years 3,856 4,067 39.7Due after ten years 2,306 2,473 24.1Total $9,657 $10,257 100.0%       Actual maturities may differ from contractual maturities when there is a right to call or prepay obligations with or without call or prepayment penalties. 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 September 30, Nine months ended September 30, 2016 2015 2016 2015Investment income:       Interest$111 $108 $330 $319Dividends39 37 117 108Other 1 1 2 2Total151 146 449 429Less investment expenses3 3 7 7Total$148 $143 $442 $422        Realized investment gains and losses summary:       Fixed maturities:       Gross realized gains$10 $4 $17 $14Gross realized losses— — (1) —Other-than-temporary impairments— (8) (2) (11)Equity securities:       Gross realized gains47 6 147 106Gross realized losses— — (1) (1)Other-than-temporary impairments— — — (1)Other(1) 1 1 3Total$56 $3 $161 $110        Change in unrealized investment gains and losses:       Fixed maturities$(20) $(15) $274 $(153)Equity securities82 (350) 367 (574)Income tax (provision) benefit(21) 127 (224) 254Total$41 $(238) $417 $(473)         During the three months ended September 30, 2016, there were no equity securities and no fixed-maturity securities other-than-temporarily impaired. During the nine months ended September 30, 2016, there were no equity securities and four fixed-maturity securities other-than-temporarily impaired. There were no credit losses on fixed-maturity securities for which a portion of other-than-temporary impairment (OTTI) has been recognized in other comprehensive income for the three and nine months ended September 30, 2016 and 2015. At September 30, 2016, 36 fixed-maturity investments with a total unrealized loss of $10 million had been in an unrealized loss position for 12 months or more. Of that total, one fixed-maturity investment had a fair value below 70 percent of amortized cost. At September 30, 2016, three equity investments with a total unrealized loss of $55 million had been in an unrealized loss position for 12 months or more. Of that total, one equity investment with a total unrealized loss of $36 million had a fair value below 70 percent of amortized cost. During 2015, we other-than-temporarily impaired 20 securities. At December 31, 2015, 69 fixed-maturity investments with a total unrealized loss of $37 million had been in an unrealized loss position for 12 months or more. Of that total, five fixed-maturity investments had fair values below 70 percent of amortized cost. There were no equity security investments in an unrealized loss position for 12 months or more as of December 31, 2015. |

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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
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