Stock-based Compensation (Summary of Restricted Stock Activity) (Details) - $ / shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, beginning of period, Shares	298,528	269,063
Granted, Shares	143,565	128,235
Vested, Shares	(71,556)	(70,590)
Forfeited, Shares	(16,890)	(13,830)
Outstanding, end of period, Shares	353,647	312,878
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 83.45	$ 73.91
Granted, Weighted Average Grant Date Fair Value	111.21	90.51
Vested, Weighted Average Grant Date Fair Value	72.56	59.29
Forfeited, Weighted Average Grant Date Fair Value	89.89	84.30
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 96.61	$ 83.55
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, beginning of period, Shares	102,586	115,057
Granted, Shares	34,280	60,101
Vested, Shares	(14,032)	(17,642)
Forfeited, Shares	(7,625)	(2,180)
Outstanding, end of period, Shares	115,209	155,336
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 81.21	$ 78.82
Granted, Weighted Average Grant Date Fair Value	117.65	79.48
Vested, Weighted Average Grant Date Fair Value	70.24	58.16
Forfeited, Weighted Average Grant Date Fair Value	81.91	95.36
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 93.34	$ 81.19

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)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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.

+ References

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-Topic 718
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-Section 50
-Paragraph 2
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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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-Publisher FASB
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-Paragraph 2
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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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- Definition

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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-Publisher FASB
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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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

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-Publisher FASB
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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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