| **Quarterly Supplementary Data (Tables)**                 | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| **Quarterly Financial Information Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Unaudited Quarterly Financial Information                 | This table includes unaudited quarterly financial information for the years ended December 31, 2014 and 2013:(Dollars in millions except per share data) Quarter    1st 2nd 3rd 4th Full year2014          Revenues  $1,189 $1,214 $1,280 $1,262 $4,945Income before income taxes 119 107 259 236 721Net income 91 84 183 167 525Net income per common share—basic 0.56 0.51 1.12 1.03 3.21Net income per common share—diluted 0.55 0.51 1.11 1.02 3.18           2013          Revenues  $1,103 $1,104 $1,152 $1,172 $4,531Income before income taxes 217 148 182 167 714Net income  154 110 131 122 517Net income per common share—basic 0.95 0.67 0.80 0.75 3.16Net income per common share—diluted 0.94 0.66 0.79 0.74 3.12            |

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| - Definition
Tabular disclosure of the quarterly financial data in the annual financial statements. The disclosure includes financial information for each fiscal quarter for the current and previous year, including revenues, gross profit, income (loss) before extraordinary items and cumulative effect of a change in accounting principle and earnings per share data.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a)-(j) -URI http://asc.fasb.org/extlink&oid=35737889&loc=d3e1280-108306
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