Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Stock Options (Details) - Stock Options - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average expected life (in years)	6 years	6 years
Weighted average exercise price (in dollars per share)	$ 158.23	$ 104.95
Expected stock price volatility (as a percent)	27.35%	30.67%
Weighted average risk–free rate	2.69%	2.13%
Weighted average fair value (in dollars per share)	$ 51.27	$ 35.58

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Weighted average expected volatility rate of share-based compensation awards.

+ References

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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.

+ References

No definition available.

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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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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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 (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Fair Value Assumptions, Weighted Average Fair Value

+ References

No definition available.

+ Details

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