BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Utility										$ 731,865	$ 695,637	$ 594,346
Nonutility										2,183,244	1,572,980	1,286,559
Total operating revenues		$ 647,326	$ 543,435	$ 1,019,043	$ 705,305	$ 536,520	$ 457,523	$ 733,546	$ 541,028	2,915,109	2,268,617	1,880,905
Depreciation and amortization										85,701	81,841	72,748
Interest income										614	2,034	128
Interest expense, net of capitalized interest										46,286	44,886	31,044
Income tax (benefit) provision										(53,785)	18,343	23,530
Equity in earnings of affiliates										13,008	13,813	9,515
Net financial earnings (loss)										240,486	149,392	138,085
Capital expenditures										384,588	328,083	356,092
Investments in equity investees										16,151	27,070	11,176
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues										2,916,044	2,222,712	1,845,139
Depreciation and amortization										85,167	81,250	71,893
Interest income	[1]									4,228	2,756	1,737
Interest expense, net of capitalized interest										49,231	45,788	31,616
Income tax (benefit) provision										(65,394)	14,129	21,519
Net financial earnings (loss)										244,642	143,214	135,837
Capital expenditures										383,375	325,649	354,196
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility										731,865	695,637	594,346
Depreciation and amortization										53,208	49,347	47,828
Interest income	[1]									614	555	115
Interest expense, net of capitalized interest										25,299	25,818	19,930
Income tax (benefit) provision										(1,910)	43,485	34,951
Net financial earnings (loss)										84,048	86,930	76,104
Capital expenditures										254,523	176,249	205,133
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility										71,375	64,394	53,540
Depreciation and amortization										31,877	31,834	23,971
Interest expense, net of capitalized interest										18,320	16,263	10,304
Income tax (benefit) provision										(79,932)	(31,161)	(26,592)
Net financial earnings (loss)										75,849	24,873	28,393
Capital expenditures										123,421	149,400	149,063
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	[2]									2,064,477	1,462,365	1,187,754
Depreciation and amortization	[3]									76	63	88
Interest income	[1]									240	6	98
Interest expense, net of capitalized interest										3,945	2,747	1,095
Income tax (benefit) provision										24,996	(4,015)	7,030
Net financial earnings (loss)										60,378	18,554	21,934
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization										6	6	6
Interest income	[1]									3,374	2,195	1,524
Interest expense, net of capitalized interest										1,667	960	287
Income tax (benefit) provision										(8,548)	5,820	6,130
Equity in earnings of affiliates										16,165	17,797	13,936
Net financial earnings (loss)										24,367	12,857	9,406
Capital expenditures										5,431	0	0
Investments in equity investees										16,151	27,070	11,176
Intercompany
Segment Reporting Information [Line Items]
Nonutility										2,665	3,370	3,232
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility										48,327	316	9,499
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility										47,392	46,221	45,265
Depreciation and amortization										780	798	981
Interest income										1,476	590	397
Interest expense, net of capitalized interest										7	410	252
Income tax (benefit) provision										11,944	3,857	1,387
Net financial earnings (loss)										(3,829)	6,811	2,882
Capital expenditures										1,213	2,434	1,896
Eliminations
Segment Reporting Information [Line Items]
Nonutility										(50,992)	(3,686)	(12,731)
Depreciation and amortization										(246)	(207)	(126)
Interest income										(5,090)	(1,312)	(2,006)
Interest expense, net of capitalized interest										(2,952)	(1,312)	(824)
Income tax (benefit) provision										(335)	357	624
Equity in earnings of affiliates										(3,157)	(3,984)	(4,421)
Net financial earnings (loss)										$ (327)	$ (633)	$ (634)

[1] Included in other income, net on the Consolidated Statements of Operations.

[2] Includes sales to Canada, which are immaterial.

[3] The amortization of acquired wholesale energy contracts is excluded above and is included in gas purchases - nonutility on the Unaudited Condensed Consolidated Statements of Operations.