STOCK-BASED COMPENSATION - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) component tranche shares
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting units ranges		0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting units ranges		200.00%
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The period from the grant date after which options expire		10 years
Total intrinsic value of options exercised	$ 0.3	$ 7.9
Unrecognized compensation cost	3.9	$ 3.9
Weighted average period over which unrecognized compensation cost is expected to be recognized		2 years 1 month 6 days
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period over which unrecognized compensation cost is expected to be recognized		2 years 4 months 24 days
Vesting period		4 years
Unrecognized compensation cost	41.8	$ 41.8
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period over which unrecognized compensation cost is expected to be recognized		1 year 10 months 24 days
Vesting period		3 years
Unrecognized compensation cost	$ 6.1	$ 6.1
Performance components \| component		2
First Component Of Performance Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The period from the grant date after which options expire		3 years
Second Component Of Performance Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The period from the grant date after which options expire		3 years
Number of tranches \| tranche		3
Sharebased payment award by performance cycle		1 year
2016 Plan \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares) \| shares		0
2016 Plan \| Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares) \| shares		474,775
2016 Plan \| Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares) \| shares		145,153

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- Definition

Number of tranches in a share based payment arrangement.

+ References

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Represents the performance cycle of each tranche in a 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.

+ References

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-Name Accounting Standards Codification
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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cost to be recognized for option 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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- Definition

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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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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- Definition

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

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- Definition

Percentage of vesting of award under share-based payment arrangement.

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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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