Share-Based Compensation Plans (Narrative) (Details)	6 Months Ended
Jun. 30, 2018 USD ($) Director $ / shares shares	Jun. 30, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $	$ 0
Stock options awarded	0	0
Aggregate intrinsic value \| $	$ 222,000	$ 277,000
Proceeds from stock options exercised \| $	$ 309,000	$ 379,000
Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued in lieu of cash	2,724
Number of non-employee directors \| Director	5
Grant date fair value \| $ / shares	$ 33.90
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares of common stock awarded	52,130
Grant date fair value \| $ / shares	$ 28.84
Unrecognized compensation expense \| $	$ 2,300,000
Expected recognition expense period, weighted average period in years	2 years
TSR Performance Requirement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	2 years 10 months 2 days
Risk free interest rate	2.39%
Expected dividend yield	2.83%
Expected stock price volatility	21.20%
Management Stock Incentive Plan [Member] \| Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Director Stock Incentive Plan [Member] \| Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares of common stock awarded	7,370
Grant date fair value \| $ / shares	$ 33.90
Required service period	1 year
Director Stock Incentive Plan [Member] \| Restricted Stock Awards [Member] \| Vested Immediately [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares of common stock awarded	3,690
Director Stock Incentive Plan [Member] \| Restricted Stock Awards [Member] \| Vested After Completion of One-Year Service Requirement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares of common stock awarded	3,680

Number of non employee directors.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2A
-Subparagraph (a)
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-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 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 estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Number of shares issued in lieu of cash for services contributed to the entity. Number of shares includes, but is not limited to, shares issued for services contributed by vendors and founders.

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