REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Utility									$ 710,793	$ 731,865	$ 695,637
Nonutility									1,881,252	2,183,244	1,572,980
Total operating revenues	$ 479,081	$ 434,942	$ 866,255	$ 811,767	$ 647,326	$ 543,435	$ 1,019,043	$ 705,305	2,592,045	2,915,109	2,268,617
Depreciation and amortization									91,730	85,701	81,841
Interest income									1,623	614	2,034
Interest expense, net of capitalized interest									47,082	46,286	44,886
Income tax provision (benefit)									(37,751)	(53,785)	18,343
Equity in earnings of affiliates									13,628	13,008	13,813
Net financial earnings (loss)									174,960	240,486	149,392
Capital expenditures									521,154	384,588	328,083
Investments in equity investees									4,102	16,151	27,070
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues									2,551,683	2,916,044	2,222,712
Depreciation and amortization									91,101	85,167	81,250
Interest income									5,072	4,228	2,756
Interest expense, net of capitalized interest									48,370	49,231	45,788
Income tax provision (benefit)									(38,806)	(65,394)	14,129
Net financial earnings (loss)									173,142	244,642	143,214
Capital expenditures									518,670	383,375	325,649
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility									710,793	731,865	695,637
Depreciation and amortization									57,980	53,208	49,347
Interest income									994	614	555
Interest expense, net of capitalized interest									26,134	25,299	25,818
Income tax provision (benefit)									9,434	(1,910)	43,485
Net financial earnings (loss)									78,062	84,048	86,930
Capital expenditures									340,226	254,523	176,249
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility									98,099	71,375	64,394
Depreciation and amortization									32,997	31,877	31,834
Interest expense, net of capitalized interest									14,846	18,320	16,263
Income tax provision (benefit)									(48,921)	(79,932)	(31,161)
Net financial earnings (loss)									77,473	75,849	24,873
Capital expenditures									157,828	123,421	149,400
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility									1,734,553	2,064,477	1,462,365
Depreciation and amortization									118	76	63
Interest income									78	240	6
Interest expense, net of capitalized interest									5,205	3,945	2,747
Income tax provision (benefit)									(1,573)	24,996	(4,015)
Net financial earnings (loss)									2,918	60,378	18,554
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization									6	6	6
Interest income									4,000	3,374	2,195
Interest expense, net of capitalized interest									2,185	1,667	960
Income tax provision (benefit)									2,254	(8,548)	5,820
Equity in earnings of affiliates									15,832	16,165	17,797
Net financial earnings (loss)									14,689	24,367	12,857
Capital expenditures									20,616	5,431	0
Investments in equity investees									4,102	16,151	27,070
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility									48,600	47,392	46,221
Depreciation and amortization									914	780	798
Interest income									1,942	1,476	590
Interest expense, net of capitalized interest									1,535	7	410
Income tax provision (benefit)									1,428	11,944	3,857
Net financial earnings (loss)									1,911	(3,829)	6,811
Capital expenditures									2,484	1,213	2,434
Intercompany
Segment Reporting Information [Line Items]
Nonutility									2,302	2,665	3,370
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility									8,238	48,327	316
Eliminations
Segment Reporting Information [Line Items]
Nonutility									(10,540)	(50,992)	(3,686)
Depreciation and amortization									(285)	(246)	(207)
Interest income									(5,391)	(5,090)	(1,312)
Interest expense, net of capitalized interest									(2,823)	(2,952)	(1,312)
Income tax provision (benefit)									(373)	(335)	357
Equity in earnings of affiliates									(2,204)	(3,157)	(3,984)
Net financial earnings (loss)									$ (93)	$ (327)	$ (633)