SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 470 ON THE UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Assets
Nonutility plant and equipment, at cost	$ 1,121,155		$ 1,108,512
Accumulated depreciation and amortization, nonutility plant and equipment	(156,063)		(140,562)
Property, plant and equipment, net	3,915,156		3,722,769
Other noncurrent assets	79,952		85,657
Total noncurrent assets	974,750		971,376
Total assets	5,425,134		5,316,477
CAPITALIZATION
Retained earnings	1,114,382		947,501
Common stock equity	1,821,833	$ 1,698,214	1,643,896	$ 1,654,737	$ 1,608,092	$ 1,345,615
Total capitalization	4,087,035		3,903,362
Liabilities
Deferred income taxes	166,238		138,081
Total noncurrent liabilities	922,033		879,393
Total capitalization and liabilities	$ 5,425,134		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	1,889,107	1,827,803	1,551,717
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)	$ (234,370)	$ (219,711)	$ (206,102)
Total capitalization			(200,796)
Liabilities
Deferred income taxes			(52,529)
Total noncurrent liabilities			(52,529)
Total capitalization and liabilities			$ (253,325)