PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) rig $ / shares	Sep. 30, 2018 USD ($) rig $ / shares	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($) rig $ / shares	Sep. 30, 2017 USD ($)	Jun. 29, 2019 USD ($) rig
Property, Plant and Equipment [Line Items]
Net book value		$ 4,857,382	$ 4,502,084	$ 4,857,382	$ 5,001,051
Asset impairment charge			224,327	23,128	0
Asset impairment charge, net of tax	$ 173,200	$ 17,200
Asset impairment charge, net of tax, per diluted share (in dollars per share) | $ / shares	$ 1.58	$ 0.16
Depreciation			556,900	578,400	584,400
Abandonments included in depreciation			11,400	27,700	42,600
Expected change in depreciation expense in future periods:
Gain on sale of assets			39,691	22,660	$ 20,627
Change in the Estimated Useful Lives of Rig Components
Property, Plant and Equipment [Line Items]
Increase in depreciation expense			4,700
Expected change in depreciation expense in future periods:
2020			800
2021			800
2022			600
2023			300
2024			300
Thereafter			500
Domestic Flex Rig4 Asset Group
Property, Plant and Equipment [Line Items]
Number of drill rigs evaluated for impairment | rig	20					71
Net book value	$ 107,500					$ 317,800
International FlexRig4 Asset Group
Property, Plant and Equipment [Line Items]
Number of drill rigs evaluated for impairment | rig	8					10
Net book value	$ 47,800					$ 55,700
FlexRig4 Asset Group
Property, Plant and Equipment [Line Items]
Net book value						$ 235,300
Asset impairment charge			224,300
Asset impairment charge, net of tax			$ 195,000
Asset impairment charge, net of tax, per diluted share (in dollars per share) | $ / shares			$ 1.78
FlexRig4 Asset Group | U.S. Land
Property, Plant and Equipment [Line Items]
Asset impairment charge			$ 216,900
Asset impairment charge, net of tax			$ 188,600
Asset impairment charge, net of tax, per diluted share (in dollars per share) | $ / shares			$ 1.72
FlexRig4 Asset Group | International Land
Property, Plant and Equipment [Line Items]
Asset impairment charge			$ 7,400
Asset impairment charge, net of tax			$ 6,400
Asset impairment charge, net of tax, per diluted share (in dollars per share) | $ / shares			$ 0.06
Decommissioned Rigs
Property, Plant and Equipment [Line Items]
Impairment charge for asset held for use				5,700
Impairment charge for asset held for use, net of tax				$ 4,200
Impairment charge for asset held for use, net of tax, per diluted share (in dollars per share) | $ / shares				$ 0.04
Six Conventional Rigs Asset Group
Property, Plant and Equipment [Line Items]
Net book value of certain rig components transferred		$ 8,500		$ 8,500
Impairment charge for assets held for sale				9,200
Impairment charge for assets held for sale, net of tax				$ 7,000
Impairment charge for assets held for sale, net of tax, per diluted share (in dollars per share) | $ / shares				$ 0.06
Impairment charge for asset held for use				$ 1,400
Impairment charge for asset held for use, net of tax				$ 1,000
Impairment charge for asset held for use, net of tax, per diluted share (in dollars per share) | $ / shares				$ 0.01
Six Conventional Rigs Asset Group | International Land
Property, Plant and Equipment [Line Items]
Net book value		$ 20,800		$ 20,800
Number of rigs to be transferred | rig		6		6