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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Narrative (Details)
 3 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
greographical_area
Mar. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Summary of Significant Accounting Policies [Line Items]            
Cash, cash equivalents, and short-term investments $ 381,700,000   $ 381,700,000      
Cash and cash equivalents 336,089,000 $ 243,912,000 336,089,000 $ 243,912,000 $ 347,943,000 $ 284,355,000
Restricted cash 44,100,000 34,200,000 44,100,000 34,200,000 35,000,000.0 $ 41,800,000
Restricted cash from initial capitalization of captive insurance company 2,000,000.0   2,000,000.0   2,000,000  
Additional cash restricted at the election of management for potential insurance claims 39,100,000   39,100,000   30,000,000.0  
Restricted cash from other purposes 1,100,000   1,100,000      
Underwriting expenses 6,000,000.0   $ 14,700,000      
Cumulative inflation rate before a country is considered highly inflationary (as a percent)     100.00%      
Aggregate foreign currency gain (loss) $ (3,400,000) $ (700,000) $ (2,800,000) $ (4,600,000)    
Stop-loss program, maximum amount of claim     $ 50,000      
Minimum            
Summary of Significant Accounting Policies [Line Items]            
Rental contract period 1 year   1 year      
Insurance coverage deductibles range for claims which occur outside or inside the United States     $ 1,000,000      
Number of geographical areas operating in (more than) | greographical_area     1      
Maximum            
Summary of Significant Accounting Policies [Line Items]            
Rental contract period 15 years   15 years      
Insurance coverage deductibles range for claims which occur outside or inside the United States     $ 10,000,000      
Intercompany Eliminations            
Summary of Significant Accounting Policies [Line Items]            
Premium revenues and expenses $ 10,500,000   18,200,000      
United States            
Summary of Significant Accounting Policies [Line Items]            
Cash and cash equivalents 294,600,000   294,600,000      
International Locations            
Summary of Significant Accounting Policies [Line Items]            
Cash and cash equivalents $ 41,500,000   $ 41,500,000      
International Locations | Operating Revenue | Geographic Concentration Risk            
Summary of Significant Accounting Policies [Line Items]            
Concentration percentage 8.20% 7.10% 8.00% 8.10%    
Argentina            
Summary of Significant Accounting Policies [Line Items]            
Cash $ 33,700,000   $ 33,700,000      
International Land | South America | International Locations | Segment Operating Revenue | Geographic Concentration Risk            
Summary of Significant Accounting Policies [Line Items]            
Concentration percentage 66.40% 93.90% 76.20% 90.90%    
Acquisition of Drilling Technology Companies            
Summary of Significant Accounting Policies [Line Items]            
Restricted cash $ 1,900,000   $ 1,900,000   $ 3,000,000.0