Stock-Based Compensation - Summary of Employee and Director Share Grant Activity (Details) - Restricted Stock - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Grants
Balance at the beginning of the period (in shares)	266,475	299,585	318,409
Granted (in shares)	85,066	95,392	119,931
Released (in shares)	(116,656)	(120,323)	(128,808)
Cancelled (in shares)	(11,771)	(8,179)	(9,947)
Balance at the end of the period (in shares)	223,114	266,475	299,585
Weighted-Average Grant-Date Fair
Balance at the beginning of the period (in dollars per share)	$ 74.76	$ 70.57	$ 55.75
Granted (in dollars per share)	86.14	74.49	87.61
Released (in dollars per share)	72.38	63.95	50.05
Cancelled (in dollars per share)	80.96	77.25	67.36
Balance at the end of the period (in dollars per share)	$ 80.02	$ 74.76	$ 70.57

No definition available.

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- Definition

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

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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.

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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).

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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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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