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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.

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

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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STOCK-BASED COMPENSATION - Narrative (Details)	12 Months Ended
STOCK-BASED COMPENSATION - Narrative (Details)	Sep. 30, 2019 USD ($) component year tranche $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 34,292,000	$ 31,687,000		$ 26,183,000
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested	200.00%
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period over which awards expire	10 years
Stock-based compensation expense	$ 3,721,000	$ 7,913,000		7,439,000
Shares granted (in shares) \| shares	0	694,000	396,000
Vesting period	1 year
Percent of options vesting annually	25.00%
Number of consecutive years that options vest \| year	4
Weighted-average fair value of options granted (in dollars per share) \| $ / shares		$ 13.17	$ 20.48
Weighted-average remaining life of exercisable stock options	4 years 5 months 19 days
Aggregate intrinsic value of exercisable stock options	$ 400,000
Weighted-average exercise price of exercisable stock options (in dollars per share) \| $ / shares	$ 60.38
Options vested or expected to vest (in shares) \| shares	755,761,000,000
Aggregate intrinsic value of options vested or expected to vest	$ 0
Weighted-average exercise price of options vested or expected to vest (in dollars per share) \| $ / shares	$ 62.42
Unrecognized compensation cost	$ 3,200,000
Weighted-average compensation cost recognition period	2 years
Intrinsic value of options exercised	$ 7,900,000	$ 9,900,000	$ 13,100,000
Grant date fair value of shares vested	8,000,000.0	8,800,000	$ 6,700,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 26,149,000	23,774,000		18,744,000
Weighted-average compensation cost recognition period	2 years 3 months 18 days
Unrecognized compensation cost related to unvested awards	$ 34,900,000
Restricted Stock \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4,422,000	$ 0		$ 0
Vesting period	3 years
Weighted-average compensation cost recognition period	1 year 10 months 24 days
Unrecognized compensation cost related to unvested awards	$ 4,700,000
Number of components included in award units \| component	2
First Component of Performance Share Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period over which awards expire	3 years
Second Component of Performance Share Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period over which awards expire	3 years
Performance cycle period of each tranche	1 year
Number of tranches in component \| tranche	3
2016 Plan \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) \| shares	0
2016 Plan \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) \| shares	474,775,000,000
2016 Plan \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) \| shares	145,153,000,000
Contract Drilling Service Operating Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 7,500,000
Selling, General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 26,800,000