STOCK-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, minimum	3 years
Number of shares of common stock that may be issued (in shares)	1,000,000
Stock Option Awards [Abstract]
Total unrecognized compensation costs	$ 8.9
Weighted average period for recognition of unrecognized compensation costs	1 year 9 months 18 days
Restricted Stock Awards
Restricted Stock Awards [Abstract]
Vested restricted stock withheld for tax withholding obligations (in shares)	35,514	30,712	31,688
Vested restricted stock withheld for tax withholding obligations, weighted average price (in dollars per share)	$ 63.83	$ 57.29	$ 50.65
Fair value, restricted stock	$ 8.9	$ 6.3	$ 6.4
Actual tax benefit realized	$ 3.3	$ 2.4	$ 2.4
Restricted Stock Awards \| Performance Vesting
Restricted Stock Awards [Abstract]
Vested (in shares)	78,825
Restricted Stock Awards \| Performance Vesting \| Executive Officers and Key Employees
Restricted Stock Awards [Abstract]
Granted, weighted average grant date fair value (in dollars per share)		$ 45.95	$ 36.69
Restricted Stock Awards \| Time Vesting
Restricted Stock Awards [Abstract]
Granted (in shares)	44,090
Granted, weighted average grant date fair value (in dollars per share)	$ 63.15
Vested (in shares)	58,200
Restricted Stock Awards \| Time Vesting \| Executive Officers and Key Employees
Restricted Stock Awards [Abstract]
Granted (in shares)	33,240
Granted, weighted average grant date fair value (in dollars per share)	$ 63.05	59.40	50.90
Restricted Stock Awards \| Time Vesting \| Non Employee Directors
Restricted Stock Awards [Abstract]
Granted (in shares)	10,850
Granted, weighted average grant date fair value (in dollars per share)	$ 63.45	$ 63.93	$ 54.66
Stock Option Awards
Stock Option Awards [Abstract]
Cash received from the exercise of stock options		$ 0.7	$ 1.5
Intrinsic value of exercised		$ 0.7	$ 0.9

Disclosure of the aggregate tax benefit realized from the exercise of equity instruments during the annual period.

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This element represents the number of vested shares withheld pursuant to elections by employees to satisfy tax withholding obligations.

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The weighted-average price of vested compensation awards withheld to satisfy tax withholding obligations.

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Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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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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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).

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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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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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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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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