FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value (Details) - Recurring Fair Value Measurements - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Assets:
Total short-term investments	$ 148,377	$ 198,700
Non-qualified supplemental savings plan	17,710	18,221
Equity and debt securities	22,812	14,358
Equity/ Cornerstone investment in ADNOC Drilling	164,783	100,000
Total investments	205,305	132,579
Liabilities
Contingent consideration	2,996	2,996
Corporate debt securities
Assets:
Total short-term investments	141,655	192,950
U.S. government and federal agency securities
Assets:
Total short-term investments	6,722	5,750
Level 1
Assets:
Total short-term investments	6,722	5,750
Non-qualified supplemental savings plan	17,710	18,221
Equity and debt securities	19,312	13,858
Equity/ Cornerstone investment in ADNOC Drilling	164,783	100,000
Total investments	201,805	132,079
Liabilities
Contingent consideration	0	0
Level 1 | Corporate debt securities
Assets:
Total short-term investments	0	0
Level 1 | U.S. government and federal agency securities
Assets:
Total short-term investments	6,722	5,750
Level 2
Assets:
Total short-term investments	141,655	192,950
Non-qualified supplemental savings plan	0	0
Equity and debt securities	0	0
Equity/ Cornerstone investment in ADNOC Drilling	0	0
Total investments	0	0
Liabilities
Contingent consideration	0	0
Level 2 | Corporate debt securities
Assets:
Total short-term investments	141,655	192,950
Level 2 | U.S. government and federal agency securities
Assets:
Total short-term investments	0	0
Level 3
Assets:
Total short-term investments	0	0
Non-qualified supplemental savings plan	0	0
Equity and debt securities	3,500	500
Equity/ Cornerstone investment in ADNOC Drilling	0	0
Total investments	3,500	500
Liabilities
Contingent consideration	2,996	2,996
Level 3 | Corporate debt securities
Assets:
Total short-term investments	0	0
Level 3 | U.S. government and federal agency securities
Assets:
Total short-term investments	$ 0	$ 0