| **Quarterly Financial Information (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** | 3 Months Ended | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
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| **Disclosure Unaudited Quarterly Financial Information [Abstract]**                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                            | $ 1,172        | $ 1,152         | $ 1,104 | $ 1,103 | $ 1,070 | $ 1,035 | $ 1,020 | $ 986 | $ 4,531 | $ 4,111 | $ 3,803 |
| Income (loss) before income taxes                                                                                   | 167            | 182             | 148 | 217 | 273 | 152 | 29 | 112 | 714 | 566 | 173 |
| Net income (loss)                                                                                                   | $ 122          | $ 131           | $ 110 | $ 154 | $ 192 | $ 111 | $ 32 | $ 86 | $ 517 | $ 421 | $ 164 |
| Net income (loss) per common share-basic                                                                            | $ 0.75         | $ 0.80          | $ 0.67 | $ 0.95 | $ 1.18 | $ 0.69 | $ 0.20 | $ 0.53 | $ 3.16 | $ 2.59 | $ 1.01 |
| Net income (loss) per common share-diluted                                                                          | $ 0.74         | $ 0.79          | $ 0.66 | $ 0.94 | $ 1.17 | $ 0.68 | $ 0.20 | $ 0.53 | $ 3.12 | $ 2.57 | $ 1.01 |

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| - Definition
The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.
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| - Definition
The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.
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| - Definition
Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.
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| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
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