Stock-based Compensation (Summary of Activity Information about Employee Restricted Stock Units) (Details) - $ / shares	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020
Time-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	350,480	335,166
Granted, Shares	170,473	142,412
Vested, Shares	(114,274)	(85,880)
Forfeited, Shares	(15,765)	(17,004)
Outstanding, end of period, Shares	390,914	374,694
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 116.37	$ 109.55
Granted, Weighted Average Grant Date Fair Value	115.32	117.72
Vested, Weighted Average Grant Date Fair Value	110.81	91.98
Forfeited, Weighted Average Grant Date Fair Value	117.01	116.00
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 117.52	$ 116.39
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	97,043	86,252
Granted, Shares	62,143	54,415
Vested, Shares	(43,506)	(40,601)
Forfeited, Shares	(1,832)	(3,023)
Outstanding, end of period, Shares	113,848	97,043
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 119.59	$ 110.70
Granted, Weighted Average Grant Date Fair Value	114.66	105.10
Vested, Weighted Average Grant Date Fair Value	122.27	80.92
Forfeited, Weighted Average Grant Date Fair Value	116.90	124.78
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 115.92	$ 119.59

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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 (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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).

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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).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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