REPORTING SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Utility	$ 317,064	$ 295,546	$ 526,851	$ 481,102
Nonutility	701,979	438,000	1,197,497	793,472
Total operating revenues	1,019,043	733,546	1,724,348	1,274,574
Depreciation and amortization	22,460	20,328	44,314	39,588
Interest income	194	108	250	183
Interest expense, net of capitalized interest	11,798	11,436	23,703	22,051
Income tax (benefit) provision	30,901	23,932	(19,267)	25,950
Equity in earnings of affiliates	3,193	5,079	6,457	7,390
Net financial earnings	142,062	104,106	277,395	144,489
Capital expenditures	97,848	78,209	164,938	164,020
Investments in equity investees	3,975	5,902	$ 11,177	$ 10,538
Canada
Segment Reporting Information [Line Items]
Percentage to total operating revenues			0.01%	1.50%
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues	1,055,243	728,776	$ 1,757,007	$ 1,259,080
Depreciation and amortization	22,298	20,205	44,031	39,293
Interest income	1,008	609	1,791	1,146
Interest expense, net of capitalized interest	12,455	11,577	24,765	22,352
Income tax (benefit) provision	32,365	23,522	(29,019)	24,995
Net financial earnings	144,640	103,670	287,784	142,734
Capital expenditures	96,002	77,864	161,779	163,504
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility	317,064	295,546	526,851	481,102
Depreciation and amortization	13,353	12,263	26,136	24,293
Interest income	131	97	250	172
Interest expense, net of capitalized interest	6,523	6,392	13,059	13,216
Income tax (benefit) provision	17,991	30,499	29,695	45,386
Net financial earnings	60,442	60,233	94,551	90,581
Capital expenditures	55,397	34,744	102,787	73,599
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility	12,866	12,943	26,862	20,510
Depreciation and amortization	8,928	7,923	17,863	14,964
Interest expense, net of capitalized interest	4,344	4,055	8,552	7,379
Income tax (benefit) provision	(12,722)	(24,756)	(86,710)	(36,643)
Net financial earnings	10,051	22,743	81,301	25,585
Capital expenditures	40,605	43,120	58,992	89,905
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility	681,475	417,608	1,153,646	756,538
Depreciation and amortization	15	17	29	33
Interest income	106	0	106	0
Interest expense, net of capitalized interest	1,203	716	2,460	1,287
Income tax (benefit) provision	23,965	16,277	37,708	13,101
Net financial earnings	72,832	15,746	93,106	19,233
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization	2	2	3	3
Interest income	771	512	1,435	974
Interest expense, net of capitalized interest	385	414	694	470
Income tax (benefit) provision	3,131	1,502	(9,712)	3,151
Equity in earnings of affiliates	4,068	6,119	8,197	9,450
Net financial earnings	1,315	4,948	18,826	7,335
Investments in equity investees	3,975	5,902	11,177	10,538
Intercompany
Segment Reporting Information [Line Items]
Nonutility	623	1,055	1,229	2,086
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility	43,838	2,679	49,648	930
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility	7,638	7,449	16,989	16,424
Depreciation and amortization	189	199	377	420
Interest income	354	132	558	253
Interest expense, net of capitalized interest	21	123	111	197
Income tax (benefit) provision	(1,281)	1,066	10,417	821
Net financial earnings	(2,488)	708	(10,204)	2,250
Capital expenditures	1,846	345	3,159	516
Eliminations
Segment Reporting Information [Line Items]
Nonutility	(44,461)	(3,734)	(50,877)	(3,016)
Depreciation and amortization	(27)	(76)	(94)	(125)
Interest income	(1,168)	(633)	(2,099)	(1,216)
Interest expense, net of capitalized interest	(678)	(264)	(1,173)	(498)
Income tax (benefit) provision	(183)	(656)	(665)	134
Equity in earnings of affiliates	(875)	(1,040)	(1,740)	(2,060)
Net financial earnings	$ (90)	$ (272)	$ (185)	$ (495)