REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT ASSETS TO CONSOLIDATED (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 5,396,905	$ 5,316,477
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	5,483,687	5,435,389
Operating Segments | Natural Gas Distribution
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	3,531,162	3,531,477
Operating Segments | Clean Energy Ventures
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	838,586	814,277
Operating Segments | Energy Services
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	259,463	244,836
Operating Segments | Storage and Transportation
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	854,476	844,799
Home Services and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	156,544	138,375
Intercompany Assets
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ (243,326)	$ (257,287)