| **Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions** | Total    | Common Stock | Additional Paid-in Capital | Retained Earnings | Accumulated Other Comprehensive Income (Loss) | Treasury Stock | Notes Receivable for the Issuance of Common Stock |
|-----------------------------------------------------------------------------|----------|--------------|----------------------------|-------------------|-----------------------------------------------|----------------|---------------------------------------------------|
| Total stockholder's equity at beginning of period at Dec. 31, 2014          |     | $ 683        | $ 2,151                    | $ 9,645           | $ 400                                         | $ (84)         | $ (1)                                             |
| **Increase (Decrease) in Stockholders' Equity [Roll Forward]**              |     |         |                       |              |                                          |           |                                              |
| Stock-based compensation                                                    |     |         | 2                          |              |                                          | 6              |                                              |
| Dividends paid to common stockholders                                       |     |         |                       | (811)             |                                          |           |                                              |
| Net income                                                                  | $ 479    |         |                       | 479               |                                          |           |                                              |
| Other comprehensive income (loss)                                           | (715)    |         |                       |              | (715)                                         |           |                                              |
| Decrease in notes receivable for common stock                               |     |         |                       |              |                                          |           | 1                                                 |
| Total stockholder's equity at end of period at Dec. 31, 2015                | 11,756   | 683          | 2,153                      | 9,313             | (315)                                         | (78)           | 0                                                 |
| **Increase (Decrease) in Stockholders' Equity [Roll Forward]**              |     |         |                       |              |                                          |           |                                              |
| Stock-based compensation                                                    |     |         | 20                         |              |                                          | 5              |                                              |
| Dividends paid to common stockholders                                       |     |         |                       | (813)             |                                          |           |                                              |
| Net income                                                                  | 859      |         |                       | 859               |                                          |           |                                              |
| Other comprehensive income (loss)                                           | 142      |         |                       |              | 142                                           |           |                                              |
| Decrease in notes receivable for common stock                               |     |         |                       |              |                                          |           | 0                                                 |
| Total stockholder's equity at end of period at Dec. 31, 2016                | 11,969   | 683          | 2,173                      | 9,359             | (173)                                         | (73)           | 0                                                 |
| **Increase (Decrease) in Stockholders' Equity [Roll Forward]**              |     |         |                       |              |                                          |           |                                              |
| Stock-based compensation                                                    |     |         | 2                          |              |                                          | 13             |                                              |
| Dividends paid to common stockholders                                       |     |         |                       | (844)             |                                          |           |                                              |
| Net income                                                                  | 899      |         |                       | 899               |                                          |           |                                              |
| Other comprehensive income (loss)                                           | 205      |         |                       |              | 205                                           |           |                                              |
| Decrease in notes receivable for common stock                               |     |         |                       |              |                                          |           | $ 0                                               |
| Total stockholder's equity at end of period at Dec. 31, 2017                | $ 12,244 | $ 683        | $ 2,175                    | $ 9,414           | $ 32                                          | $ (60)         |                                              |

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Change in equity related to changes in Notes Receivable for Common Stock
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| - Definition
Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
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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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| - Definition
Amount after tax and reclassification adjustments of other comprehensive income (loss).
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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