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

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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 equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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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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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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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STOCK-BASED COMPENSATION - Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restricted stock awards
Number of Shares
Beginning balance (in shares)			657,240
Granted (in shares)			79,060
Vested (in shares)			(207,032)
Forfeited (in shares)			(48,166)
Ending balance (in shares)	481,102		481,102
Weighted Average Grant-Date Fair Value
Beginning of period (in USD per share)			$ 54.65
Granted (in USD per share)			65.12
Vested (in USD per share)			53.75
Forfeited (in USD per share)			54.92
End of period (in USD per share)	$ 56.73		$ 56.73
Unrecognized compensation cost	$ 21.3		$ 21.3
Period for recognition (in years)			2 years 4 months 24 days
Fair value restricted stock	$ 7.9	$ 7.0	$ 11.1	$ 8.3
Minimum
Weighted Average Grant-Date Fair Value
Vesting period (in years)			3 years
Maximum
Weighted Average Grant-Date Fair Value
Vesting period (in years)			5 years