The cumulative number of shares that have been repurchased and have not been retired and are not held in treasury.

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The cumulative equity impact of the value of stock that has been repurchased and has not been retired and is not held in treasury.

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The maximum number of shares originally approved for issuance under the Employee Stock Purchase and Bonus Plan.

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The difference between the maximum number of shares authorized for issuance under the Employee Stock Purchase and Bonus Plan and the number of shares already issued under the Employee Stock Purchase and Bonus Plan.

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The carrying amount as of the balance sheet date of outstanding subsidized unsecured loans made to employees and directors that participate in the Company's Employee Stock Purchase and Bonus Plan.

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The Company's Employee Stock Purchase and Bonus Plan includes a loan program that allows participants to purchase shares of the Company's common stock with a subsidized unsecured loan. Bonus shares are awarded to participants equal to 5% of the shares acquired through the plan's loan program in a given year.

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The Company's Employee Stock Purchase and Bonus Plan encourages share ownership by providing for the award of bonus shares to participants equal to 10% of the net increase in the number of shares owned under the plan in a given year, excluding shares acquired through the plan's loan program component.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan.

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Number of shares issuable in exchange for each vested Restricted Stock Unit

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Face amount or stated value per share of common stock.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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-Subparagraph (i)
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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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 stock repurchase plan authorized.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized	50,000,000	50,000,000
Common stock, par value	$ 0	$ 0
Common stock, shares issued	13,794,142	13,887,711
Common stock, shares outstanding	13,794,142	13,887,711
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0	$ 0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Stock Repurchased During Period, Shares	104,719
Stock Repurchased During Period, Value	$ 111.6
May 2018 Share Repurchase Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchased During Period, Shares	73,371
Stock Repurchased During Period, Value	$ 75.8
Cumulative Stock Repurchased Shares	93,036
Cumulative stock repurchased value	$ 97.2
August 2019 Share Repurchase Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Board of Directors approved the repurchase of common stock under a share repurchase program, maximum	$ 300.0
Stock Repurchased During Period, Shares	31,348
Stock Repurchased During Period, Value	$ 35.8
2016 Employee Stock Purchase And Bonus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Bonus calculation percentage based on the value of stocks acquired by employees	10.00%
Percentage of bonus shares award to loan program participants	5.00%
Shares authorized under the 2016 Employee Stock Purchase and Bonus Plan	125,000
Number of shares available for purchase	100,471	105,283
Loans outstanding under 2016 Employee Stock Purchase and Bonus Plan	$ 18.4	$ 19.2
2016 Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future awards	205,686
2016 Compensation Plan [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted stock units awarded	15,145
Number of restricted stock units vested	17,391
Weighted average grant-date fair value, share-based awards	$ 1,025.81	$ 1,121.68	$ 979.23
Unrecognized compensation cost related to nonvested share-based awards	$ 7.8
Weighted average remaining service period of share-based awards	1 year 7 months 6 days
Fair value of the vested share-based awards	$ 17.6	$ 19.1	$ 28.8
2016 Compensation Plan [Member] \| Restricted Stock Units (RSUs) [Member] \| Non-Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted stock units awarded	968
2016 Compensation Plan [Member] \| Restricted Stock Units (RSUs) [Member] \| Performance Based [Member] \| Associates And Executive Officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issuable for each vested restricted stock unit	1
Number of restricted stock units awarded	11,418
Restricted stock awards, vesting period	3 years
2016 Compensation Plan [Member] \| Restricted Stock Units (RSUs) [Member] \| Retention And Hiring [Member] \| Associates And Executive Officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issuable for each vested restricted stock unit	1
Number of restricted stock units awarded	2,759