STOCK-BASED COMPENSATION - Performance Stock Awards (Details) - Performance stock awards $ / shares in Units, $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)
Number of Shares
Beginning balance (in shares) \| shares	213,490
Granted (in shares) \| shares	0
Vested (in shares) \| shares	(51,996)
Forfeited (in shares) \| shares	(23,554)
Ending balance (in shares) \| shares	137,940
Weighted Average Grant-Date Fair Value
Beginning of period (in USD per share) \| $ / shares	$ 55.71
Granted (in USD per share) \| $ / shares	0
Vested (in USD per share) \| $ / shares	55.77
Forfeited (in USD per share) \| $ / shares	55.64
End of period (in USD per share) \| $ / shares	$ 55.69
Unrecognized compensation cost \| $	$ 3.4
Fair value restricted stock \| $	$ 2.9	$ 0.5

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

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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).

+ References

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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).

+ References

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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