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Earnings Per Share - Calculation of Average Share Outstanding (Details) - $ / shares shares in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |
Weighted average exercise price of options (usd per share) | $ 26.79 | $ 33.32 | $ 26.93
Mandatory convertible preferred stock |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |
Number of anti-dilutive securities (in shares) |  | 39 | 39
Convertible notes |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |
Number of anti-dilutive securities (in shares) | 0 | 14 | 14
Stock Options |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |
Number of anti-dilutive securities (in shares) | 9 | 11 | 13
Stock awards |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |
Number of anti-dilutive securities (in shares) | 0 | 7 | 8

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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