SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 470 ON THE UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Assets
Nonutility plant and equipment, at cost	$ 1,117,839	$ 1,108,512
Accumulated depreciation and amortization, nonutility plant and equipment	(148,666)	(140,562)
Property, plant and equipment, net	3,818,315	3,722,769
Other noncurrent assets	81,393	85,657
Total noncurrent assets	969,386	971,376
Total assets	5,396,905	5,316,477
CAPITALIZATION
Retained earnings	996,580	947,501
Common stock equity	1,698,214	1,643,896	$ 1,608,092	$ 1,345,615
Capitalization, Long-term Debt and Equity	3,963,186	3,903,362
Liabilities
Deferred income taxes	166,128	138,081
Total noncurrent liabilities	915,047	879,393
Liabilities and Equity	$ 5,396,905	5,316,477
As Previously Reported
Assets
Nonutility plant and equipment, at cost		1,430,723
Accumulated depreciation and amortization, nonutility plant and equipment		(202,507)
Property, plant and equipment, net		3,983,035
Other noncurrent assets		78,716
Total noncurrent assets		964,435
Total assets		5,569,802
CAPITALIZATION
Retained earnings		1,148,297
Common stock equity		1,844,692	1,827,803	1,551,717
Capitalization, Long-term Debt and Equity		4,104,158
Liabilities
Deferred income taxes		190,610
Total noncurrent liabilities		931,922
Liabilities and Equity		5,569,802
Effect of Change
Assets
Nonutility plant and equipment, at cost		(322,211)
Accumulated depreciation and amortization, nonutility plant and equipment		61,945
Property, plant and equipment, net		(260,266)
Other noncurrent assets		6,941
Total noncurrent assets		6,941
Total assets		(253,325)
CAPITALIZATION
Retained earnings		(200,796)
Common stock equity		(200,796)	$ (219,711)	$ (206,102)
Capitalization, Long-term Debt and Equity		(200,796)
Liabilities
Deferred income taxes		(52,529)
Total noncurrent liabilities		(52,529)
Liabilities and Equity		$ (253,325)