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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, including forfeitures.

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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.

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Amount of cost for defined contribution plan.

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Unrecognized cost of unvested share-based compensation awards.

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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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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 equity instruments other than options outstanding, including both vested and non-vested instruments.

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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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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Intrinsic value of equity-based compensation awards outstanding. Excludes stock and unit options.

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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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Purchase price of common stock expressed as a percentage of its fair value.

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STOCK-BASED COMPENSATION PLANS - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
STOCK-BASED COMPENSATION PLANS:
Options outstanding (in shares)	375,000
Weighted average exercise price of options (in dollars per share)	$ 31.08
Weighted average remaining contractual life of options	8 years
Aggregate intrinsic value of options	$ 2.5
Stock Based Compensation Plans
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 18.5	$ 16.9	$ 18.0
ESPP
STOCK-BASED COMPENSATION PLANS:
Number of shares reserved for award (in shares)	3,200,000
Compensation expense	$ 1.0	0.9	0.7
Number of shares available for future grant (in shares)	850,876
ESPP \| Maximum
STOCK-BASED COMPENSATION PLANS:
Percentage of the fair market value of common stock as of the first day of the quarter or on last day of the quarter	85.00%
RSAs
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 3.2	2.8	5.3
Unrecognized compensation expense	$ 2.0
Unrestricted shares granted (in shares)	103,955
Stock Grants \| Non Employee Director
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 0.7	$ 0.6	$ 0.6
Unrestricted shares granted (in shares)	20,000	20,000	20,000
SARs
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 6.6	$ 4.0	$ 2.6
SARs term	10 years
SAR's outstanding (in shares)	2,610,000	2,310,000
SAR's outstanding intrinsic value	$ 38.6
SAR's remaining contractual life	6 years 110 days
Stock Options
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 0.0	$ 0.4	0.8
401 (K) Plan
STOCK-BASED COMPENSATION PLANS:
Compensation expense relating to match	$ 7.3	6.9	6.2
Number of shares reserved for matches (in shares)	3,000,000
Number of shares available for future grants (in shares)	242,951
Subsidiary Stock Awards
STOCK-BASED COMPENSATION PLANS:
Compensation expense	$ 0.0	$ 1.3	$ 1.8
Class A Common Stock \| LTIP
STOCK-BASED COMPENSATION PLANS:
Number of shares reserved for award (in shares)	14,000,000
Number of shares (including forfeited shares) available for future grants	6,487,654
Award Date, 2017 \| RSAs \| LTIP
STOCK-BASED COMPENSATION PLANS:
Vesting period	2 years
Percentage of restriction to be lapsed in year one from grant date	50.00%
Percentage of restriction to be lapsed in year two from grant date	50.00%
Award Date, 2016 \| RSAs \| LTIP
STOCK-BASED COMPENSATION PLANS:
Vesting period	2 years
Percentage of restriction to be lapsed in year one from grant date	50.00%
Percentage of restriction to be lapsed in year two from grant date	50.00%
Award Date, 2015 \| RSAs \| LTIP
STOCK-BASED COMPENSATION PLANS:
Vesting period	2 years
Percentage of restriction to be lapsed in year one from grant date	50.00%
Percentage of restriction to be lapsed in year two from grant date	50.00%