REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT ASSETS TO CONSOLIDATED (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 5,425,134	$ 5,316,477
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	5,540,965	5,435,389
Operating Segments | Natural Gas Distribution
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	3,593,630	3,531,477
Operating Segments | Clean Energy Ventures
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	848,290	814,277
Operating Segments | Energy Services
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	224,069	244,836
Operating Segments | Storage and Transportation
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	874,976	844,799
Home Services and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	136,531	138,375
Intercompany Assets
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ (252,362)	$ (257,287)