IMMATERIAL RESTATEMENT OF PRIOR PERIOD FINANCIAL STATEMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Capitalization:
Retained earnings	$ 556,698	$ 514,873	$ 461,146	$ 406,083
Total equity	1,322,394	1,171,917	910,281	768,843
Total capitalization	2,374,881	2,227,711
Current liabilities:
Regulatory balancing accounts	12,240	32,908
Total current liabilities	294,650	271,937
Deferred income taxes	$ 330,251	294,647
As Previously Reported
Capitalization:
Retained earnings		525,936	472,209	417,146
Total equity		1,182,980	921,344	779,906
Total capitalization		2,238,774
Current liabilities:
Regulatory balancing accounts		17,547
Total current liabilities		256,576
Deferred income taxes		298,945
Corrections
Capitalization:
Retained earnings		(11,063)	(11,063)	(11,063)
Total equity		(11,063)	$ (11,063)	$ (11,063)
Total capitalization		(11,063)
Current liabilities:
Regulatory balancing accounts		15,361
Total current liabilities		15,361
Deferred income taxes		$ (4,298)