REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Utility	$ 209,787	$ 185,556
Nonutility	495,518	355,472
Total operating revenues	705,305	541,028
Depreciation and amortization	21,854	19,260
Interest income	56	75
Interest expense, net of capitalized interest	11,905	10,615
Income tax (benefit) provision	(50,168)	2,018
Equity in earnings of affiliates	3,264	2,311
Net financial earnings	135,333	40,383
Capital expenditures	67,090	85,811
Investments in equity investees	$ 7,202	$ 4,636
Canada
Segment Reporting Information [Line Items]
Percentage to total operating revenues	0.02%	1.90%
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	$ 701,764	$ 530,304
Depreciation and amortization	21,733	19,088
Interest income	783	537
Interest expense, net of capitalized interest	12,310	10,775
Income tax (benefit) provision	(61,384)	1,473
Net financial earnings	143,144	39,064
Capital expenditures	65,777	85,640
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility	209,787	185,556
Depreciation and amortization	12,783	12,030
Interest income	119	75
Interest expense, net of capitalized interest	6,536	6,824
Income tax (benefit) provision	11,704	14,887
Net financial earnings	34,109	30,348
Capital expenditures	47,390	38,855
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility	13,996	7,567
Depreciation and amortization	8,935	7,041
Interest expense, net of capitalized interest	4,208	3,324
Income tax (benefit) provision	(73,988)	(11,887)
Net financial earnings	71,250	2,842
Capital expenditures	18,387	46,785
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	472,171	338,930
Depreciation and amortization	14	16
Interest expense, net of capitalized interest	1,257	571
Income tax (benefit) provision	13,743	(3,176)
Net financial earnings	20,274	3,487
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization	1	1
Interest income	664	462
Interest expense, net of capitalized interest	309	56
Income tax (benefit) provision	(12,843)	1,649
Equity in earnings of affiliates	4,129	3,331
Net financial earnings	17,511	2,387
Investments in equity investees	7,202	4,636
Intercompany
Segment Reporting Information [Line Items]
Nonutility	606	1,031
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility	5,810	(1,749)
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility	9,351	8,975
Depreciation and amortization	188	221
Interest income	204	121
Interest expense, net of capitalized interest	90	74
Income tax (benefit) provision	11,698	(245)
Net financial earnings	(7,716)	1,542
Capital expenditures	1,313	171
Eliminations
Segment Reporting Information [Line Items]
Nonutility	(6,416)	718
Depreciation and amortization	(67)	(49)
Interest income	(931)	(583)
Interest expense, net of capitalized interest	(495)	(234)
Income tax (benefit) provision	(482)	790
Equity in earnings of affiliates	(865)	(1,020)
Net financial earnings	$ (95)	$ (223)