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Common Stockholders' Equity and Preferred Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule Of Stockholders Equity [Line Items] |  |  |
Number of shares repurchased | 28,000,000 |  |
Repurchases of common stock | $ 1,844 | $ 2,607 | $ 1,232
Share repurchase program additional amount authorized | 3,000 |  |
Amount remaining under repurchase programs | $ 3,000 |  |
Treasury stock, shares, retired | 25,000,000 |  |
Treasury stock retired, average cost per share | $ 63 |  |
Common stock reserved for future issuance | 10,000,000 |  |
Junior preferred stock outstanding | 2 |  |
Junior preferred stock par value per share | $ 0.01 |  |
Preferred stock authorized to issue | 250,000,000 |  |

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