BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Utility		$ 242,536	$ 374,703	$ 394,142	$ 583,430
Nonutility		331,657	638,387	624,309	1,253,784
Total operating revenues		574,193	1,013,090	1,018,451	1,837,214
Depreciation and amortization		17,744	15,204	34,226	29,590
Interest income	[1]	53	246	123	336
Interest expense, net of capitalized interest		7,369	6,483	14,146	13,678
Income tax provision (benefit)		17,840	20,144	26,197	61,011
Equity in earnings of affiliates		2,402	3,587	4,808	6,382
Net financial earnings		77,880	99,143	127,512	154,253
Capital expenditures		73,514	66,318	168,357	158,422
Investments in equity investees		3,102	717	$ 5,948	$ 1,264
Canada [Member]
Segment Reporting Information [Line Items]
Percentage to total operating revenues				2.90%	4.40%
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total operating revenues		570,156	1,060,975	$ 1,008,249	$ 1,879,625
Depreciation and amortization		17,499	14,968	33,871	29,128
Interest income	[1]	510	287	914	616
Interest expense, net of capitalized interest		7,540	6,547	14,431	13,736
Income tax provision (benefit)		18,130	20,478	27,241	62,292
Net financial earnings		80,000	100,524	130,433	156,274
Capital expenditures		73,242	66,268	167,288	158,361
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization		11,598	10,647	22,836	21,192
Interest income	[1]	60	10	128	100
Interest expense, net of capitalized interest		4,690	4,388	9,278	8,997
Income tax provision (benefit)		22,710	23,778	36,573	37,944
Net financial earnings		48,961	48,594	79,531	76,780
Capital expenditures		47,366	33,243	96,406	69,991
Operating Segments [Member] | Natural Gas Distribution [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Utility		242,536	374,703	394,142	583,430
Operating Segments [Member] | Energy Services [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization		23	23	46	45
Interest income	[1]	0	13	72	13
Interest expense, net of capitalized interest		168	288	376	801
Income tax provision (benefit)		7,805	3,923	13,000	49,800
Net financial earnings		17,005	36,316	27,019	52,752
Operating Segments [Member] | Energy Services [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	[2]	316,703	626,107	592,885	1,226,669
Operating Segments [Member] | Energy Services [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility		3,255	56,097	5,766	59,223
Operating Segments [Member] | Clean Energy Ventures [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization		5,876	4,297	10,986	7,888
Interest income	[1]	0	22	0	22
Interest expense, net of capitalized interest		2,552	1,661	4,605	3,478
Income tax provision (benefit)		(13,915)	(9,042)	(25,502)	(28,763)
Net financial earnings		11,806	13,010	19,311	22,018
Capital expenditures		25,876	33,025	70,882	88,370
Operating Segments [Member] | Clean Energy Ventures [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility		7,662	4,068	15,456	10,303
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization		2	1	3	3
Interest income	[1]	450	242	714	481
Interest expense, net of capitalized interest		130	210	172	460
Income tax provision (benefit)		1,530	1,819	3,170	3,311
Equity in earnings of affiliates		3,508	4,581	7,053	8,356
Net financial earnings		2,228	2,604	4,572	4,724
Investments in equity investees		3,102	717	5,948	1,264
Corporate, Non-Segment [Member] | Home Services and Other [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization		237	238	464	476
Interest income	[1]	123	201	160	201
Interest expense, net of capitalized interest		105	(64)	105	(58)
Income tax provision (benefit)		(1,456)	(972)	(1,744)	(1,424)
Net financial earnings		(2,040)	(1,359)	(2,623)	(1,951)
Capital expenditures		272	50	1,069	61
Corporate, Non-Segment [Member] | Home Services and Other [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility		7,292	8,212	15,968	16,812
Corporate, Non-Segment [Member] | Home Services and Other [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility		639	375	1,536	786
Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility		(3,894)	(56,472)	(7,302)	(60,009)
Depreciation and amortization		8	(2)	(109)	(14)
Interest income	[1]	(580)	(242)	(951)	(481)
Interest expense, net of capitalized interest		(276)	0	(390)	0
Income tax provision (benefit)		1,166	638	700	143
Equity in earnings of affiliates		(1,106)	(994)	(2,245)	(1,974)
Net financial earnings		$ (80)	$ (22)	$ (298)	$ (70)

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

[2] Includes sales to Canada, which accounted for 2.9 and 4.4 percent of total operating revenues during the six months ended March 31, 2016 and 2015.