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LONG-TERM DEBT - Revolving Credit Facility (Details) - Revolving Credit Facility
12 Months Ended
Sep. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Sep. 30, 2016
USD ($)
Debt Instrument [Line Items]      
Required ratio of total priority debt to EBITDA through period end 2.25    
Unused capacity, commitment fee percentage 0.45%    
Amount outstanding $ 0   $ 0
Amount outstanding, letters of credit $ 0   $ 0
LIBOR      
Debt Instrument [Line Items]      
Basis spread on variable rate 3.00%    
Revolving Credit Facility with Conversion Election Agreement      
Debt Instrument [Line Items]      
Maximum borrowing capacity   $ 525,000,000  
Deferred issuance costs   $ 4,000,000  
Amended Revolving Credit Facility      
Debt Instrument [Line Items]      
Collateral maximum test value 1.0    
Value of company assets collateralized $ 749,000,000    
Maximum limit on issuance of letters of credit $ 100,000,000    
Overnight Revolving Credit Loans | Prime Rate      
Debt Instrument [Line Items]      
Basis spread on variable rate 2.00%