SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Utility Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Depreciable plant and equipment
Equipment	$ 4,557,964	$ 4,215,619
Office buildings and other structures	$ 32,921	27,779
Office Buildings and other structures, useful life	50 years
Minimum
Depreciable plant and equipment
Equipment, useful life	5 years
Transmission and distribution plant, useful life	40 years
Maximum
Depreciable plant and equipment
Equipment, useful life	50 years
Transmission and distribution plant, useful life	65 years
Utility Plant
Depreciable plant and equipment
Equipment	$ 976,704	915,322
Office buildings and other structures	354,628	339,682
Transmission and distribution plant	3,226,632	2,960,615
Total	$ 4,557,964	$ 4,215,619