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Earnings Per Share | 12 Months Ended
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Dec. 31, 2017
Earnings Per Share |
Earnings Per Share | 15. Earnings Per Share Basic and diluted earnings per share were computed using the following common share data for the years ended December 31 (shares in millions):
|  | 2017 |  | 2016 |  | 2015
Number of common shares outstanding at year-end |  | 433.3 |  | 439.3 |  | 447.2
Effect of using weighted average common shares outstanding |  | 5.5 |  | 4.2 |  | 5.5
Weighted average basic common shares outstanding |  | 438.8 |  | 443.5 |  | 452.7
Dilutive effect of equity-based compensation awards and other contingently issuable shares (a) |  | 3.1 |  | 3.0 |  | 3.2
Weighted average diluted common shares outstanding |  | 441.9 |  | 446.5 |  | 455.9
Potentially issuable shares |  | 8.1 |  | 9.8 |  | 10.2
Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding |  | 1.9 |  | 1.0 |  | 2.0
| (a) |  | As of January 1, 2017, we adopted ASU 2016‑09 prospectively and no longer include excess tax benefits as assumed proceeds. See Note 2 for further discussion.

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- Definition The entire disclosure for earnings per share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -URI http://asc.fasb.org/topic&trid=2144383 + Details
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