STOCK-BASED COMPENSATION - Stock option activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Number of Shares
Beginning balance (in shares)	442,042
Exercised (in shares)	(40,133)
Ending balance (in shares)	401,909	442,042
Weighted Average Exercise Price
Beginning balance (in dollars per share)	$ 25.36
Exercised (in dollars per share)	19.26
Ending balance (in dollars per share)	$ 25.97	$ 25.36
Weighted Average Remaining Contractual Term (years)
Outstanding		1 year
Outstanding and expected to vest	5 years
Aggregate Intrinsic Value (in millions)
Beginning balance	$ 17.5
Ending balance	$ 31.4	$ 17.5
Exercisable
Number of shares (in shares)	361,374
Weighted average exercise price (in dollars per share)	$ 25.73
Weighted average remaining contractual term (years)	4 years 10 months 24 days
Aggregate intrinsic value	$ 28.3

Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value [Abstract]

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Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Remaining Contractual Term [Abstract]

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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-Publisher FASB
-Name Accounting Standards Codification
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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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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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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