| **Investments**            | 3 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 March 31, 2016  gains losses Fixed maturity securities:        Corporate  $5,395 $295 $66 $5,624States, municipalities and political subdivisions 3,499 204 — 3,703Commercial mortgage-backed  289 11 2 298Government-sponsored enterprises 240 — — 240Foreign government 10 — — 10Convertibles and bonds with warrants attached 5 — — 5United States government 4 — — 4Subtotal 9,442 510 68 9,884Equity securities:        Common equities 2,810 1,954 44 4,720Nonredeemable preferred equities 189 33 1 221Subtotal 2,999 1,987 45 4,941Total $12,441 $2,497 $113 $14,825At 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 March 31, 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), BlackRock Inc. (NYSE:BLK), Johnson and Johnson (NYSE:JNJ), Hasbro Incorporated (Nasdaq:HAS), and Microsoft Corporation (Nasdaq:MSFT), which had a combined gross unrealized gain of $597 million. At March 31, 2016, Apple was our largest single common stock holding with a fair value of $153 million, whichwas3.2 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 March 31, 2016      Fixed maturity securities:            Corporate  $801 $43 $153 $23 $954 $66States, municipalities and political subdivisions 28 — 4 — 32 —Commercial mortgage-backed  49 1 2 1 51 2Government-sponsored enterprises 29 — 19 — 48 —Subtotal 907 44 178 24 1,085 68Equity securities:            Common equities 345 44 — — 345 44Nonredeemable preferred equities 42 1 — — 42 1Subtotal 387 45 — — 387 45Total $1,294 $89 $178 $24 $1,472 $113At 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 March 31, 2016   Maturity dates:      Due in one year or less $481 $488 4.9%Due after one year through five years 3,017 3,202 32.4Due after five years through ten years 3,676 3,797 38.4Due after ten years 2,268 2,397 24.3Total $9,442 $9,884 100.0%       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 March 31, 2016 2015Investment income:   Interest$109 $105Dividends37 36Other 1 —Total147 141Less investment expenses2 2Total$145 $139    Realized investment gains and losses summary:   Fixed maturities:   Gross realized gains$3 $3Gross realized losses(1) —Other-than-temporary impairments(2) —Equity securities:   Gross realized gains62 44Gross realized losses(1) (1)Other-than-temporary impairments— —Other— 1Total$61 $47    Change in unrealized investment gains and losses:   Fixed maturities$116 $46Equity securities174 (89)Less income taxes(100) 15Total$190 $(28)     During the three months ended March 31, 2016, there were no equity securities and two 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 months ended March 31, 2016 and 2015. At March 31, 2016, 44 fixed-maturity investments with a total unrealized loss of $24 million had been in an unrealized loss position for 12 months or more. Of that total, two fixed-maturity investments had fair values below 70 percent of amortized cost. There were no equity investments in an unrealized loss position for 12 months or more as of March 31, 2016. 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.
+ References
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