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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Schedule of Gross Receipts Taxes [Line Items]    
Gross Receipts taxes recorded in regulated gas distribution operating revenues $ 17.9 $ 27.7
Laclede Gas    
Schedule of Gross Receipts Taxes [Line Items]    
Gross Receipts taxes recorded in regulated gas distribution operating revenues 13.9 21.9
Alagasco    
Schedule of Gross Receipts Taxes [Line Items]    
Gross Receipts taxes recorded in regulated gas distribution operating revenues $ 4.0 $ 5.8