REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Utility		$ 301,420	$ 317,064	$ 501,385	$ 526,851
Nonutility		564,835	701,979	1,176,637	1,197,497
Total operating revenues		866,255	1,019,043	1,678,022	1,724,348
Depreciation and amortization		22,311	22,460	44,143	44,314
Interest income	[1]	419	194	618	250
Interest expense, net of capitalized interest		12,509	11,798	25,995	23,703
Income tax provision (benefit)		(2,952)	30,901	(9,913)	(19,267)
Equity in earnings of affiliates		3,371	3,193	6,532	6,457
Net financial earnings	[2]	112,417	142,062	166,510	277,395
Capital expenditures		103,101	97,848	197,402	164,938
Investments in equity investees		1,457	3,975	1,457	11,177
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues		860,605	1,055,243	1,662,734	1,757,007
Depreciation and amortization		22,090	22,298	43,937	44,031
Interest income	[1]	1,140	1,008	2,395	1,791
Interest expense, net of capitalized interest		12,608	12,455	26,569	24,765
Income tax provision (benefit)		(1,945)	32,365	(8,713)	(29,019)
Net financial earnings		114,078	144,640	168,017	287,784
Capital expenditures		98,046	96,002	191,616	161,779
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility		301,420	317,064	501,385	526,851
Depreciation and amortization		13,972	13,353	27,868	26,136
Interest income	[1]	181	131	380	250
Interest expense, net of capitalized interest		5,762	6,523	11,865	13,059
Income tax provision (benefit)		12,266	17,991	18,096	29,695
Net financial earnings		68,546	60,442	100,259	94,551
Capital expenditures		79,341	55,397	139,096	102,787
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility		11,360	12,866	26,257	26,862
Depreciation and amortization		8,091	8,928	16,014	17,863
Interest expense, net of capitalized interest		4,657	4,344	10,085	8,552
Income tax provision (benefit)		(14,042)	(12,722)	(37,246)	(86,710)
Net financial earnings		21,730	10,051	31,935	81,301
Capital expenditures		20,394	40,605	52,520	58,992
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	[3]	541,794	681,475	1,126,754	1,153,646
Depreciation and amortization	[4]	25	15	52	29
Interest income	[1]	14	106	14	106
Interest expense, net of capitalized interest		1,624	1,203	3,511	2,460
Income tax provision (benefit)		(1,388)	23,965	8,256	37,708
Net financial earnings		19,304	72,832	27,674	93,106
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization		2	2	3	3
Interest income	[1]	945	771	2,001	1,435
Interest expense, net of capitalized interest		565	385	1,108	694
Income tax provision (benefit)		1,219	3,131	2,181	(9,712)
Equity in earnings of affiliates		3,998	4,068	7,799	8,197
Net financial earnings		4,498	1,315	8,149	18,826
Capital expenditures		(1,689)	0	0	0
Investments in equity investees		1,457	3,975	1,457	11,177
Intercompany
Segment Reporting Information [Line Items]
Nonutility		652	623	1,197	1,229
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility		6,031	43,838	8,338	49,648
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility		11,681	7,638	23,626	16,989
Depreciation and amortization		222	189	443	377
Interest income	[1]	669	354	1,105	558
Interest expense, net of capitalized interest		611	21	1,003	111
Income tax provision (benefit)		(659)	(1,281)	(851)	10,417
Net financial earnings		(1,581)	(2,488)	(1,505)	(10,204)
Capital expenditures		5,055	1,846	5,786	3,159
Eliminations
Segment Reporting Information [Line Items]
Nonutility		(6,683)	(44,461)	(9,535)	(50,877)
Depreciation and amortization		(1)	(27)	(237)	(94)
Interest income	[1]	(1,390)	(1,168)	(2,882)	(2,099)
Interest expense, net of capitalized interest		(710)	(678)	(1,577)	(1,173)
Income tax provision (benefit)		(348)	(183)	(349)	(665)
Equity in earnings of affiliates		(627)	(875)	(1,267)	(1,740)
Net financial earnings		$ (80)	$ (90)	$ (2)	$ (185)

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

[2] Includes income tax benefit related to the Tax Act of $967,000 and $58.5 million, for the three and six months ended March 31, 2018, respectively.

[3] Includes sales to Canada, which are immaterial

[4] 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.