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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in contract assets and liabilities (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Contract assets    
Opening balance $ 1,879 $ 2,119
Additions 6,985 2,511
Deductions (2,670) (2,751)
Ending balance 6,194 1,879
Contract liabilities    
Opening balance 11,392 6,726
Additions 208,377 106,956
Deductions (193,155) (102,290)
Ending balance $ 26,614 $ 11,392