Stock-Based Compensation - Stock Option Activity (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) $ / shares shares
Stock Options, Number of Shares
Under option, January 1, 2018 (in shares) \| shares	4.5
Granted (in shares) \| shares	0.5
Exercised (in shares) \| shares	(0.4)
Under option, December 31, 2018 (in shares) \| shares	4.6
Stock Options, Weighted-Average Remaining Contractual Term
Under option, January 1, 2018 (in dollars per share) \| $ / shares	$ 80.88
Granted (in dollars per share) \| $ / shares	163.36
Exercised (in dollars per share) \| $ / shares	61.22
Under option, December 31, 2018 (in dollars per share) \| $ / shares	$ 90.56
Stock Options
Number of shares, exercisable, December 31, 2018 (in shares) \| shares	3.2
Weighted-average exercise price, exercisable, December 31, 2018 (in dollars per share) \| $ / shares	$ 74.08
Weighted-average remaining contractual term, under option, December 31, 2018	5 years 7 months 6 days
Weighted-average reamaining contractual term, exercisable, December 31, 2018	4 years 7 months 6 days
Aggregate intrinsic value, under option, December 31, 2018 \| $	$ 165
Aggregate intrinsic value, exercisable, December 31, 2018 \| $	$ 168

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Net number of share options (or share units) granted during the period.

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

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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