BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Utility	$ 121,362	$ 119,206	$ 602,464	$ 513,348
Nonutility	336,161	274,007	1,129,633	898,316
Total operating revenues	457,523	393,213	1,732,097	1,411,664
Depreciation and amortization	20,760	18,671	60,348	52,897
Interest income	159	65	342	188
Interest expense, net of capitalized interest	11,164	7,787	33,215	21,933
Income tax (benefit) provision	(5,816)	(14,190)	20,134	10,347
Equity in earnings of affiliates	3,065	2,257	10,455	7,065
Net financial earnings	17,395	10,972	161,884	140,144
Capital expenditures	74,705	81,938	238,725	250,295
Investments in equity investees	13,559	2,741	$ 24,097	$ 8,689
Canada
Segment Reporting Information [Line Items]
Percentage to total operating revenues			1.40%	2.40%
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	443,416	382,216	$ 1,702,496	$ 1,390,465
Depreciation and amortization	20,596	18,391	59,889	52,262
Interest income	789	473	1,935	1,387
Interest expense, net of capitalized interest	11,436	7,920	33,788	22,351
Income tax (benefit) provision	(6,784)	(15,946)	18,211	10,502
Net financial earnings	16,119	8,661	158,853	139,887
Capital expenditures	74,288	81,541	237,792	248,829
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility	121,362	119,206	602,464	513,348
Depreciation and amortization	12,425	12,297	36,718	35,133
Interest income	202	62	374	190
Interest expense, net of capitalized interest	6,294	4,935	19,510	14,213
Income tax (benefit) provision	1,496	2,015	46,882	38,232
Net financial earnings	5,951	3,607	96,532	83,494
Capital expenditures	42,235	36,687	115,834	133,093
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility	14,915	12,703	35,425	28,159
Depreciation and amortization	8,154	6,070	23,118	17,056
Interest expense, net of capitalized interest	4,345	2,666	11,724	7,271
Income tax (benefit) provision	(8,122)	(2,784)	(44,765)	(28,433)
Net financial earnings	6,276	2,440	31,861	21,898
Capital expenditures	32,053	44,854	121,958	115,736
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	307,448	247,633	1,063,986	840,518
Depreciation and amortization	16	23	49	69
Interest income	6	16	6	88
Interest expense, net of capitalized interest	586	263	1,873	639
Income tax (benefit) provision	(1,767)	(16,678)	11,334	(3,968)
Net financial earnings	933	276	20,166	27,585
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization	1	1	4	4
Interest income	581	395	1,555	1,109
Interest expense, net of capitalized interest	211	56	681	228
Income tax (benefit) provision	1,609	1,501	4,760	4,671
Equity in earnings of affiliates	4,049	3,359	13,499	10,412
Net financial earnings	2,959	2,338	10,294	6,910
Investments in equity investees	13,559	2,741	24,097	8,689
Intercompany
Segment Reporting Information [Line Items]
Nonutility	610	737	2,696	2,273
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility	(309)	2,674	621	8,440
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility	13,798	13,671	30,222	29,639
Depreciation and amortization	186	249	606	713
Interest income	155	97	408	257
Interest expense, net of capitalized interest	116	65	313	170
Income tax (benefit) provision	853	1,556	1,674	(1,055)
Net financial earnings	1,295	2,418	3,545	662
Capital expenditures	417	397	933	1,466
Eliminations
Segment Reporting Information [Line Items]
Nonutility	(301)	(3,411)	(3,317)	(10,713)
Depreciation and amortization	(22)	31	(147)	(78)
Interest income	(785)	(505)	(2,001)	(1,456)
Interest expense, net of capitalized interest	(388)	(198)	(886)	(588)
Income tax (benefit) provision	115	200	249	900
Equity in earnings of affiliates	(984)	(1,102)	(3,044)	(3,347)
Net financial earnings	$ (19)	$ (107)	$ (514)	$ (405)