REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT ASSETS TO CONSOLIDATED (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 4,186,928	$ 3,928,507
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	4,184,460	3,922,001
Operating Segments | Natural Gas Distribution
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,590,623	2,519,578
Operating Segments | Energy Services
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	547,388	398,277
Operating Segments | Clean Energy Ventures
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	797,951	771,340
Operating Segments | Midstream
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	248,498	232,806
Home Services and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	125,149	114,801
Intercompany Assets
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ (122,681)	$ (108,295)