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Fair value of a share of common stock, percentage

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Percentage of ownership in subsidiaries

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Percentage of total combined voting power of the Company and our subsidiaries

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Number of stock units authorized for achievement of targeted performance

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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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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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Amount of tax benefit from exercise of option under share-based payment arrangement.

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The amount of plan compensation cost recognized during the period.

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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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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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Discount rate from fair value on purchase date that participants pay for shares.

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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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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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Number of shares authorized for issuance under share-based payment arrangement.

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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 sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Description of terms of award under share-based payment arrangement.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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CAPITAL STOCK AND SHARE-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 07, 2012	Jun. 06, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, authorized (shares)	60,000,000	60,000,000
Common stock, par value (usd per share)	$ 0.001	$ 0.001
Preferred stock, authorized (shares)	5,000,000	5,000,000
Preferred stock, par value (usd per share)	$ 0.001	$ 0.001
Common stock, issued (shares)	37,470,212	36,638,021
Common stock, outstanding (shares)	32,814,278	32,284,051
Preferred stock, issued (shares)	0	0
Percentage of ownership in subsidiaries	50.00%
Equity-based awards, number of shares authorized (shares)	2,500,000
Equity-based awards, shares available for grant (shares)	2,000,000.0
Percentage of total combined voting power of the Company and subsidiaries	10.00%
Percentage of market value for purchases under Employee Stock Purchase Program (percent)	85.00%
Common stock authorized for issuance under Employee Stock Purchase Plan (shares)	1,328,627
Employee Stock Purchase Plan expense	$ 0.6	$ 0.6	$ 0.5
Options granted (shares)	43,249
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual term of share-based award	ten years
Common stock authorized for issuance under Employee Stock Purchase Plan (shares)				4,500,000	2,500,000
Vesting period of equity-based awards	4 years
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of equity-based awards	12 months
Share-Based Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share of common stock (percent)	100.00%
Share Based Awards to More Than Ten Percent Owner [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share of common stock (percent)	110.00%
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (shares)	43,249	142,122	163,666
Stock compensation expense	$ 4.3	$ 6.2	$ 5.7
Tax benefit from stock option exercise	27.9	1.3	1.6
Intrinsic value of options outstanding	50.6
Intrinsic value of options exerciseable	19.4
Intrinsic value of options exercised during the period	121.1	7.3	9.7
Unrecognized compensation expense	$ 4.8
Unrecognized compensation expense weighted-average period for recognitions (years)	1 year 10 months 24 days
Non-Vested Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 0.8	$ 1.2	$ 1.4
Non-vested stock granted, weighted average grant date fair value (usd per share)	$ 158.72	$ 119.12	$ 80.54
Vesting period of equity-based awards	1 year
Non-Vested Stock Units [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of equity-based awards	4 years
Non-Vested Stock Units [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of equity-based awards	1 year
Non-Vested Stock Units - Service-Based [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 7.5	$ 8.7	$ 4.5
Unrecognized compensation expense	$ 9.3
Unrecognized compensation expense weighted-average period for recognitions (years)	1 year 9 months 18 days
Non-vested stock granted, weighted average grant date fair value (usd per share)	$ 206.10	$ 123.70	$ 95.14
Non-Vested Stock Units - Service-Based and Performance-Based [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 13.5	$ 8.4	$ 5.8
Unrecognized compensation expense	$ 17.3
Unrecognized compensation expense weighted-average period for recognitions (years)	1 year 9 months 18 days
Non-vested stock granted, weighted average grant date fair value (usd per share)	$ 201.90	$ 128.89	$ 79.59
Performance-based award, target number of units to be received (shares)	81,183