BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Utility	$ 295,546	$ 242,536	$ 481,102	$ 394,142
Nonutility	438,000	331,657	793,472	624,309
Total operating revenues	733,546	574,193	1,274,574	1,018,451
Depreciation and amortization	20,328	17,744	39,588	34,226
Interest income	108	53	183	123
Interest expense, net of capitalized interest	11,436	7,369	22,051	14,146
Income tax provision (benefit)	23,932	17,815	25,950	24,537
Equity in earnings of affiliates	5,079	2,402	7,390	4,808
Net financial earnings	104,106	77,905	144,489	129,172
Capital expenditures	78,209	73,514	164,020	168,357
Investments in equity investees	5,902	3,102	$ 10,538	$ 5,948
Canada
Segment Reporting Information [Line Items]
Percentage to total operating revenues			1.50%	2.90%
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	728,776	570,156	$ 1,259,080	$ 1,008,249
Depreciation and amortization	20,205	17,499	39,293	33,871
Interest income	609	510	1,146	914
Interest expense, net of capitalized interest	11,577	7,540	22,352	14,431
Income tax provision (benefit)	23,522	18,123	24,995	26,441
Net financial earnings	103,670	80,007	142,734	131,233
Capital expenditures	77,864	73,242	163,504	167,288
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility	295,546	242,536	481,102	394,142
Depreciation and amortization	12,263	11,598	24,293	22,836
Interest income	97	60	172	128
Interest expense, net of capitalized interest	6,392	4,690	13,216	9,278
Income tax provision (benefit)	30,499	22,704	45,386	36,211
Net financial earnings	60,233	48,967	90,581	79,893
Capital expenditures	34,744	47,366	73,599	96,406
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility	12,943	7,662	20,510	15,456
Depreciation and amortization	7,923	5,876	14,964	10,986
Interest expense, net of capitalized interest	4,055	2,552	7,379	4,605
Income tax provision (benefit)	(24,756)	(13,916)	(36,643)	(25,650)
Net financial earnings	22,743	11,807	25,585	19,459
Capital expenditures	43,120	25,876	89,905	70,882
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	417,608	316,703	756,538	592,885
Depreciation and amortization	17	23	33	46
Interest income	0	0	0	72
Interest expense, net of capitalized interest	716	168	1,287	376
Income tax provision (benefit)	16,277	7,805	13,101	12,710
Net financial earnings	15,746	17,005	19,233	27,309
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization	2	2	3	3
Interest income	512	450	974	714
Interest expense, net of capitalized interest	414	130	470	172
Income tax provision (benefit)	1,502	1,530	3,151	3,170
Equity in earnings of affiliates	6,119	3,508	9,450	7,053
Net financial earnings	4,948	2,228	7,335	4,572
Investments in equity investees	5,902	3,102	10,538	5,948
Intercompany
Segment Reporting Information [Line Items]
Nonutility	1,055	639	2,086	1,536
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility	2,679	3,255	930	5,766
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility	7,449	7,292	16,424	15,968
Depreciation and amortization	199	237	420	464
Interest income	132	123	253	160
Interest expense, net of capitalized interest	123	105	197	105
Income tax provision (benefit)	1,066	(1,474)	821	(2,604)
Net financial earnings	708	(2,022)	2,250	(1,763)
Capital expenditures	345	272	516	1,069
Eliminations
Segment Reporting Information [Line Items]
Nonutility	(3,734)	(3,894)	(3,016)	(7,302)
Depreciation and amortization	(76)	8	(125)	(109)
Interest income	(633)	(580)	(1,216)	(951)
Interest expense, net of capitalized interest	(264)	(276)	(498)	(390)
Income tax provision (benefit)	(656)	1,166	134	700
Equity in earnings of affiliates	(1,040)	(1,106)	(2,060)	(2,245)
Net financial earnings	$ (272)	$ (80)	$ (495)	$ (298)