Stock-based Compensation (Summary of Restricted Stock Activity) (Details) - $ / shares	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Time-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	332,481	298,528
Granted, Shares	144,072	131,290
Vested, Shares	(99,855)	(50,810)
Forfeited, Shares	(8,687)	(10,585)
Outstanding, end of period, Shares	368,011	368,423
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 97.28	$ 83.45
Granted, Weighted Average Grant Date Fair Value	117.51	110.60
Vested, Weighted Average Grant Date Fair Value	83.25	70.24
Forfeited, Weighted Average Grant Date Fair Value	97.14	87.47
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 109.01	$ 94.83
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	69,838	102,586
Granted, Shares	42,119	29,715
Vested, Shares	(10,089)	(14,032)
Forfeited, Shares		(7,625)
Outstanding, end of period, Shares	101,868	110,644
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 95.58	$ 81.21
Granted, Weighted Average Grant Date Fair Value	117.68	116.82
Vested, Weighted Average Grant Date Fair Value	82.74	70.24
Forfeited, Weighted Average Grant Date Fair Value		81.91
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 105.99	$ 92.12

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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- Definition

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
-SubTopic 10
-Section 50
-Paragraph 2
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Paragraph 2
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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

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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Paragraph 2
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- Definition

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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- Definition

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

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-Publisher FASB
-Name Accounting Standards Codification
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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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-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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No definition available.

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