| **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, 2015  gains losses Fixed maturity securities:        Corporate  $5,152 $449 $8 $5,593States, municipalities and political subdivisions 3,289 183 2 3,470Commercial mortgage-backed  269 16 — 285Government-sponsored enterprises 227 — 3 224Foreign government 10 — — 10Convertibles and bonds with warrants attached 7 — — 7United States government 7 — — 7Subtotal 8,961 648 13 9,596Equity securities:        Common equities 2,595 2,014 11 4,598Nonredeemable preferred equities 153 38 — 191Subtotal 2,748 2,052 11 4,789Total $11,709 $2,700 $24 $14,385At 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          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. At March 31, 2015, we had $25 million of short-term investments, which consisted of commercial paper, that had no gross unrealized gains or losses. At December 31, 2014, we held no short-term investments. The seven largest unrealized investment gains in our common stock portfolio are from Exxon Mobil Corporation (NYSE:XOM), Honeywell International Incorporated (NYSE:HON), BlackRock Inc. (NYSE:BLK), Apple Inc. (Nasdaq:AAPL), The Procter & Gamble Company (NYSE:PG), RPM International (NYSE:RPM), and 3M Company (NYSE:MMM), which had a combined gross unrealized gain of $605 million. At March 31, 2015, Apple Inc. was our largest single common stock holding with a fair value of 3.8 percent of our publicly traded common stock portfolio and 1.2 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, 2015      Fixed maturity securities:            Corporate  $211 $6 $64 $2 $275 $8States, municipalities and political subdivisions 115 2 58 — 173 2Commercial mortgage-backed  2 — 2 — 4 —Government-sponsored enterprises 18 — 156 3 174 3Subtotal 346 8 280 5 626 13Equity securities:            Common equities 168 11 — — 168 11Nonredeemable preferred equities 5 — 15 — 20 —Subtotal 173 11 15 — 188 11Total $519 $19 $295 $5 $814 $24At 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              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, 2015 2014Investment income:   Interest$105 $104Dividends36 32Other — 1Total141 137Less investment expenses2 2Total$139 $135    Realized investment gains and losses summary:   Fixed maturities:   Gross realized gains$3 $2Gross realized losses— (1)Other-than-temporary impairments— —Equity securities:   Gross realized gains44 18Gross realized losses(1) —Other-than-temporary impairments— (1)Other1 4Total$47 $22    Change in unrealized investment gains and losses:   Fixed maturities$46 $88Equity securities(89) 29Less income taxes15 (41)Total$(28) $76     During the three months ended March 31, 2015, there were no equity securities and no 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, 2015 and 2014. At March 31, 2015, 70 fixed-maturity investments with a total unrealized loss of $5 million had been in an unrealized loss position for 12 months or more. Of that total, no fixed-maturity investments had fair values below 70 percent of amortized cost. Two equity investments with a total unrealized loss of less than $1 million had been in an unrealized loss position for 12 months or more as of March 31, 2015. Of that total, no equity investments were trading below 70 percent of cost. During 2014, we other-than-temporarily impaired six fixed-maturity securities. At December 31, 2014, 144 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, no fixed-maturity investments had fair values below 70 percent of amortized cost. There were three equity security investments in an unrealized loss position for 12 months or more with a total unrealized loss of $1 million as of December 31, 2014. Of that total, no equity security investments had fair values below 70 percent of cost. |

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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Debt Security -URI http://asc.fasb.org/extlink&oid=6509901Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Equity Security -URI http://asc.fasb.org/extlink&oid=6511694
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