Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares received for each RSU upon vesting (in shares)	1
Shares reserved for issuance under long-term incentive plans (in shares)	13,000,000
Weighted-average grant-date fair value of options granted (in dollars per share)	$ 38.34	$ 26.83	$ 20.02
Aggregate intrinsic value of options exercised	$ 33	$ 132	$ 89
Cash receipts from exercise of options	22	84	84
Total fair value of vested stock option awards	15	13	12
Total fair value of vested restricted stock unit awards	$ 19	$ 19	$ 21
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
Total unrecognized compensation cost related to unvested awards	$ 10
Weighted-average period to recognize total unrecognized compensation cost	2 years 1 month 6 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested awards	$ 30
Weighted-average period to recognize total unrecognized compensation cost	1 year 9 months 18 days
Periodic Vesting \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Cliff Vesting \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Cliff Vesting \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years

Number of shares for each vested RSU or PRSU award.

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Share based compensation arrangement by share based payment award equity instruments stock options vested in period fair value.

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- Definition

Aggregate number of common shares reserved for future issuance.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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