Stock-based Compensation (Details) - Equity Incentive Plan - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Stock-based Compensation
Recorded compensation costs for the RSAs and RSUs	$ 0.7
Restricted Stock Awards (RSAs)
Stock-based Compensation
Weighted average grant date fair value (in dollars per share)	$ 35.40	$ 36.75
Restricted Stock Awards (RSAs) \| Officers and Directors
Stock-based Compensation
Awards granted (in shares)	46,135	48,717
Awards canceled (in shares)	9,464	10,902
Restricted Stock Awards (RSAs) \| Employees
Stock-based Compensation
Vesting period	36 months
Restricted Stock Awards (RSAs) \| Director
Stock-based Compensation
Vesting period	12 months
Restricted Stock Unit Award (RSUs) \| Officer
Stock-based Compensation
Awards granted (in shares)	28,594	31,389
Weighted average grant date fair value (in dollars per share)	$ 35.40	$ 36.75
Awards issued (in shares)	48,753	38,709
Awards canceled (in shares)	24,009	19,735
Expiration period of award		3 years

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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-Publisher FASB
-Name Accounting Standards Codification
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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 equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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