BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT ASSETS TO CONSOLIDATED (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Sep. 30, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 3,857,359	$ 3,718,570
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	3,822,676	3,696,982
Operating Segments | Natural Gas Distribution
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,572,576	2,517,401
Operating Segments | Energy Services
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	328,372	327,626
Operating Segments | Clean Energy Ventures
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	709,289	665,696
Operating Segments | Midstream
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	212,439	186,259
Home Services and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	126,255	109,487
Intercompany Assets
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ (91,572)	$ (87,899)