SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
RESTRICTED CASH AND CASH EQUIVALENTS
Cash and cash equivalents	$ 228,462	$ 284,355	$ 383,664	$ 521,375
Restricted cash and cash equivalents	34,100	41,800
Total cash, cash equivalents, and restricted cash	262,590	326,185	428,688	560,509
Restricted cash and cash equivalents from initial capitalization of captive insurance company	2,000	2,000
Additional cash and cash equivalents restricted at the election of management for potential insurance claims	29,100	28,500
Drilling technology companies
RESTRICTED CASH AND CASH EQUIVALENTS
Restricted cash and cash equivalents	3,000	11,300
Prepaid expenses and other
RESTRICTED CASH AND CASH EQUIVALENTS
Restricted cash and cash equivalents	30,246	39,830	38,226	32,439
Other assets.
RESTRICTED CASH AND CASH EQUIVALENTS
Restricted cash and cash equivalents	$ 3,882	$ 2,000	$ 6,798	$ 6,695