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Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
NUMERATOR: |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ 1,888 | [1] | $ 503 | $ 746 | $ 351 | $ 522 | $ 388 | $ 820 | $ 513 | $ 3,488 | $ 2,244 | $ 2,181
Incremental income effect of interest on 5.25% convertible notes |  |  |  |  |  |  |  |  |  | 0 | 2 | 4
Net income after assumed conversion |  |  |  |  |  |  |  |  |  | $ 3,488 | $ 2,246 | $ 2,185
DENOMINATOR: |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average shares outstanding, basic |  |  |  |  |  |  |  |  |  | 601 | 627 | 661
Dilutive effect of Employee stock options and restricted stock units |  |  |  |  |  |  |  |  |  | 2 | 1 | 2
Dilutive effect of 5.25% convertible notes |  |  |  |  |  |  |  |  |  | 0 | 5 | 6
Adjusted weighted-average shares outstanding, diluted |  |  |  |  |  |  |  |  |  | 603 | 633 | 669
NET INCOME PER SHARE: |  |  |  |  |  |  |  |  |  |  |  |
Basic (in dollars per share) | $ 3.19 | [1] | $ 0.84 | $ 1.24 | $ 0.57 | $ 0.85 | $ 0.63 | $ 1.30 | $ 0.80 | $ 5.80 | $ 3.58 | $ 3.30
Diluted (in dollars per share) | $ 3.18 | [1] | $ 0.84 | $ 1.23 | $ 0.57 | $ 0.84 | $ 0.62 | $ 1.28 | $ 0.79 | $ 5.79 | $ 3.55 | $ 3.27
Convertible Debt | 5.25% Convertible Senior Notes due 2016 |  |  |  |  |  |  |  |  |  |  |  |
Potentially dilutive amounts excluded from calculations: |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.25% |  |  |  |  | 5.25% |  |  |  | 5.25% | 5.25% | 5.25%
[1] | Includes a $1.4 billion reduction in Provision for income taxes related to the Tax Cuts and Jobs Act legislation enacted in December 2017, which resulted in a re-measurement of the Company's deferred tax assets and liabilities at the new federal corporate tax rate of 21 percent. See Note 14 to the Consolidated Financial Statements for further information.

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