BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Utility									$ 695,637	$ 594,346	$ 781,970
Nonutility									1,572,980	1,286,559	1,952,017
Total operating revenues	$ 536,520	$ 457,523	$ 733,546	$ 541,028	$ 469,241	$ 393,213	$ 574,193	$ 444,258	2,268,617	1,880,905	2,733,987
Depreciation and amortization									81,841	72,748	61,399
Interest income									2,034	128	580
Interest expense, net of capitalized interest									44,886	31,044	27,721
Income tax (benefit) provision									18,343	23,530	59,724
Equity in earnings of affiliates									13,813	9,515	13,409
Net financial earnings									149,392	138,085	151,503
Capital expenditures									328,083	356,092	320,086
Investments in equity investees									$ 27,070	$ 11,176	$ 5,780
CANADA
Segment Reporting Information [Line Items]
Percentage to total operating revenues									0.80%	2.00%	3.70%
Operating Segments
Segment Reporting Information [Line Items]
Total operating revenues									$ 2,222,712	$ 1,845,139	$ 2,748,790
Depreciation and amortization									81,250	71,893	60,478
Interest income									2,756	1,737	1,777
Interest expense, net of capitalized interest									45,788	31,616	28,095
Income tax (benefit) provision									14,129	21,519	58,468
Net financial earnings									143,214	135,837	148,290
Capital expenditures									325,649	354,196	319,877
Operating Segments | Natural Gas Distribution
Segment Reporting Information [Line Items]
Utility									695,637	594,346	781,970
Depreciation and amortization									49,347	47,828	43,085
Interest income									555	115	336
Interest expense, net of capitalized interest									25,818	19,930	18,534
Income tax (benefit) provision									43,485	34,951	39,544
Net financial earnings									86,930	76,104	76,287
Capital expenditures									176,249	205,133	168,875
Operating Segments | Clean Energy Ventures
Segment Reporting Information [Line Items]
Nonutility									64,394	53,540	32,513
Depreciation and amortization									31,834	23,971	17,297
Interest income									0	0	26
Interest expense, net of capitalized interest									16,263	10,304	7,635
Income tax (benefit) provision									(31,161)	(26,592)	(26,968)
Net financial earnings									24,873	28,393	20,101
Capital expenditures									149,400	149,063	151,002
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Nonutility									1,462,365	1,187,754	1,872,781
Depreciation and amortization									63	88	90
Interest income									6	98	438
Interest expense, net of capitalized interest									2,747	1,095	1,209
Income tax (benefit) provision									(4,015)	7,030	39,043
Net financial earnings									18,554	21,934	42,122
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Depreciation and amortization									6	6	6
Interest income									2,195	1,524	977
Interest expense, net of capitalized interest									960	287	717
Income tax (benefit) provision									5,820	6,130	6,849
Equity in earnings of affiliates									17,797	13,936	17,487
Net financial earnings									12,857	9,406	9,780
Investments in equity investees									27,070	11,176	5,780
Intercompany
Segment Reporting Information [Line Items]
Nonutility									3,370	3,232	1,980
Intercompany | Energy Services
Segment Reporting Information [Line Items]
Nonutility									316	9,499	61,526
Home Services and Other
Segment Reporting Information [Line Items]
Nonutility									46,221	45,265	46,723
Depreciation and amortization									798	981	952
Interest income									590	397	217
Interest expense, net of capitalized interest									410	252	49
Income tax (benefit) provision									3,857	1,387	1,551
Net financial earnings									6,811	2,882	3,420
Capital expenditures									2,434	1,896	209
Eliminations
Segment Reporting Information [Line Items]
Nonutility									(3,686)	(12,731)	(63,506)
Depreciation and amortization									(207)	(126)	(31)
Interest income									(1,312)	(2,006)	(1,414)
Interest expense, net of capitalized interest									(1,312)	(824)	(423)
Income tax (benefit) provision									357	624	(295)
Equity in earnings of affiliates									(3,984)	(4,421)	(4,078)
Net financial earnings									$ (633)	$ (634)	$ (207)