Stock-Based Compensation (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation [Line Items]
Service period	3 years
Stock options
Share-based Compensation [Line Items]
Vesting period	3 years
Expiration period	7 years
Equity-classified awards, unrecognized compensation cost	$ 14
Weighted average period over which unrecognized cost is recognized, years	1 year 6 months
Restricted stock
Share-based Compensation [Line Items]
Vesting period	4 years
RSUs
Share-based Compensation [Line Items]
Vesting period	4 years	4 years
Weighted average period over which unrecognized cost is recognized, years	3 years
Liability classified awards, unrecognized compensation costs	$ 61
Performance units
Share-based Compensation [Line Items]
Vesting period	3 years	3 years
Weighted average period over which unrecognized cost is recognized, years	3 years
Liability classified awards, unrecognized compensation costs	$ 19

Share-based Compensation [Line Items]

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

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.

+ References

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- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

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- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

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.

+ References

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- Definition

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.

+ References

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