| **Selected Quarterly Financial Data (Unaudited) (Tables)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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| **Quarterly Financial Information Disclosure [Abstract]**  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Selected quarterly financial data                          | (Millions of dollars)      2012 First quarter Second quarter Third quarter Fourth quarterTotal revenues $668 $668 $678 $679Profit before income taxes $170 $144 $153 $124Profit $120 $104 $109 $99         2011 First quarter Second quarter Third quarter Fourth quarterTotal revenues $640 $675 $668 $662Profit before income taxes $115 $152 $126 $111Profit $83 $107 $93 $95         2010 First quarter Second quarter Third quarter Fourth quarterTotal revenues $631 $648 $640 $633Profit before income taxes $71 $95 $89 $74Profit $53 $82 $73 $70          |

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| - Details**Name:**us-gaap_QuarterlyFinancialInformationDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
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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=20225539&loc=d3e1280-108306
+ Details**Name:**us-gaap_ScheduleOfQuarterlyFinancialInformationTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |