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DERIVATIVE INSTRUMENTS - BALANCE SHEET RELATED DISCLOSURES (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Mar. 13, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	May 30, 2018	May 15, 2018	May 11, 2018	Jan. 31, 2018	Jun. 30, 2015
Fair Value
Derivative assets, current		$ 22,712		$ 22,712		$ 27,396
Derivative liabilities, current		48,019		48,019		46,652
Derivative assets, noncurrent		13,277		13,277		10,560
Derivative liabilities, noncurrent		24,927		24,927		22,982
Treasury lock
Derivatives, Fair Value [Line Items]
Benchmark interest rate											3.26%
Interest rate, stated percentage									4.01%
Gain (loss) on derivative, net	$ (2,600)
Interest rate swap
Derivatives, Fair Value [Line Items]
Benchmark interest rate										2.8375%
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative, net		1,305	$ (42,925)	(9,084)	$ (90,457)
Fair Value
Derivative assets		35,989		35,989		37,956
Derivative liabilities		72,946		72,946		69,634
Natural Gas Distribution \| Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative, net		279	6,887	7,652	(8,964)
Natural Gas Distribution \| Not Designated as Hedging Instrument \| Physical commodity contracts
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative, net		1,436	(12,879)	4,413	(15,855)
Fair Value
Derivative assets, current		91		91		85
Derivative liabilities, current		49		49		192
Natural Gas Distribution \| Not Designated as Hedging Instrument \| Financial commodity contracts
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative, net		(1,157)	7,232	3,239	(1,576)
Fair Value
Derivative assets, current		578		578		94
Derivative liabilities, current		7		7		0
Natural Gas Distribution \| Not Designated as Hedging Instrument \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative, net		0	$ 12,534	0	$ 8,467
Natural Gas Distribution \| Series Ll
Derivatives, Fair Value [Line Items]
Debt issued								$ 125,000
Interest rate, stated percentage								5.60%
Energy Services \| Not Designated as Hedging Instrument \| Physical commodity contracts
Fair Value
Derivative assets, current		4,868		4,868		7,667
Derivative liabilities, current		17,474		17,474		18,158
Derivative assets, noncurrent		1,013		1,013		3,930
Derivative liabilities, noncurrent		18,184		18,184		11,316
Energy Services \| Not Designated as Hedging Instrument \| Financial commodity contracts
Fair Value
Derivative assets, current		17,053		17,053		19,169
Derivative liabilities, current		30,279		30,279		28,176
Derivative assets, noncurrent		12,264		12,264		6,630
Derivative liabilities, noncurrent		6,599		6,599		11,548
Energy Services \| Not Designated as Hedging Instrument \| Foreign currency contracts
Fair Value
Derivative assets, current		2		2		0
Derivative liabilities, current		210		210		126
Derivative assets, noncurrent		0		0		0
Derivative liabilities, noncurrent		144		144		118
First Mortgage \| Natural Gas Distribution \| Series VV
Derivatives, Fair Value [Line Items]
Debt issued							$ 125,000		$ 125,000
Term Loan \| Credit Agreement Due August 16, 2019 \| Interest rate swap
Derivatives, Fair Value [Line Items]
Debt issued										$ 100,000
Home Services and Other \| Not Designated as Hedging Instrument \| Interest rate contracts
Fair Value
Derivative assets, current		120		120		381
Derivative liabilities, current		$ 0		$ 0		$ 0