Common Stock and Stock Compensation - Table Stock Option Activity (Details) - $ / shares shares in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at December 31, 2016	18.6	24.6
Exercised	6.0
Cancelled	0.0
Outstanding and Exercisable at December 31, 2017	18.6
Weighted Average Exercise Price
Outstanding at December 31, 2016	$ 41.68	$ 41.28
Cancelled	39.90
Exercised	40.03
Outstanding and Exercisable at December 31, 2017	$ 41.68

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

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

+ References

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- Definition

Weighted average price of options that were either forfeited or expired.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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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

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Number of share options (or share units) exercised during the current period.

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