Share-Based Compensation Plans - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	3,000	4,000
Weighted- average exercise price of options granted in period (in dollars per share)	$ 57.72	$ 63.96
Weighted- average fair value per option granted (in dollars per share)	$ 8.19	$ 7.04
Total pretax unrecognized compensation expense related to nonvested stock options	$ 654
Weighted average period in years of recognition for nonvested stock options	2 years 1 month 6 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	7,000	5,000
Weighted-average fair value per share granted (in dollars per share)	$ 58.19	$ 63.96
Performance Share Units (PSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	855	1,000
Weighted-average fair value per share granted (in dollars per share)	$ 56.70	$ 63.62

Unrecognized cost of unvested share-based compensation awards.

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.

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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-Publisher FASB
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- Definition

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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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 718
-SubTopic 10
-Section 50
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- Definition

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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-Publisher FASB
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- Definition

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

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