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STOCK-BASED COMPENSATION - Compensation cost (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Stock based awards |  |  |
Compensation expense (in dollars) | $ 31,687 | $ 26,183 | $ 24,383
Stock options |  |  |
Stock based awards |  |  |
Number of shares granted | 694,000 | 396,000 | 876,000
Compensation expense (in dollars) | $ 7,913 | $ 7,439 | $ 8,290
Restricted stock |  |  |
Stock based awards |  |  |
Number of shares granted | 626,000 | 292,000 | 294,000
Compensation expense (in dollars) | $ 23,774 | $ 18,744 | $ 16,093
2016 Plan | Stock options |  |  |
Stock based awards |  |  |
The period from the grant date after which options expire | 10 years |  |
Number of shares granted | 693,873 |  |
2016 Plan | Restricted stock |  |  |
Stock based awards |  |  |
Number of shares granted | 411,977 |  |
2010 Plan | Restricted stock |  |  |
Stock based awards |  |  |
Number of shares granted | 213,904 |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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. + 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 (a)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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