SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 470 ON THE UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Assets
Nonutility plant and equipment, at cost	$ 1,124,896	$ 1,108,512
Accumulated depreciation and amortization, nonutility plant and equipment	(171,709)	(140,562)
Property, plant and equipment, net	4,213,513	3,722,769
Other noncurrent assets	49,921	85,657
Total noncurrent assets	874,491	971,376
Total assets	5,722,278	5,316,477	$ 4,157,942
Capitalization
Retained earnings	934,610	947,501
Common stock equity	1,630,862	1,643,896	1,381,833	$ 1,294,664
Total capitalization	3,793,026	3,903,362
Liabilities
Deferred income taxes	163,530	138,081
Total noncurrent liabilities	877,842	879,393
Total capitalization and liabilities	$ 5,722,278	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	1,551,717	1,418,978
Total capitalization		4,104,158
Liabilities
Deferred income taxes		190,610
Total noncurrent liabilities		931,922
Total capitalization and liabilities		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)	$ (169,884)	$ (124,314)
Total capitalization		(200,796)
Liabilities
Deferred income taxes		(52,529)
Total noncurrent liabilities		(52,529)
Total capitalization and liabilities		$ (253,325)