PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Electric Plant
Plant in service (at cost)	$ 6,090,392	$ 5,899,128
Accumulated depreciation	(2,641,470)	(2,499,376)
Construction work in progress	164,924	207,732
Net electric plant	3,613,846	3,607,484
Electric Plant
Electric Plant
Plant in service (at cost)	6,090,392	5,899,128
Accumulated depreciation	(2,641,470)	(2,499,376)
Net Book Value	3,448,922	3,399,752
Construction work in progress	164,924	207,732
Net electric plant	3,613,846	3,607,484
Generation plant
Electric Plant
Plant in service (at cost)	3,681,886	3,601,911
Accumulated depreciation	(1,599,528)	(1,504,802)
Net Book Value	$ 2,082,358	$ 2,097,109
Generation plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	0.89%	0.89%
Generation plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	6.27%	6.27%
Transmission
Electric Plant
Plant in service (at cost)	$ 1,679,534	$ 1,556,860
Accumulated depreciation	(600,740)	(562,216)
Net Book Value	$ 1,078,794	$ 994,644
Transmission | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.11%	1.11%
Transmission | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	2.09%	2.09%
General plant
Electric Plant
Plant in service (at cost)	$ 472,592	$ 492,991
Accumulated depreciation	(321,304)	(316,233)
Net Book Value	$ 151,288	$ 176,758
General plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.46%	1.46%
General plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	9.53%	9.53%
Other
Electric Plant
Plant in service (at cost)	$ 256,380	$ 247,366
Accumulated depreciation	(119,898)	(116,125)
Net Book Value	$ 136,482	$ 131,241
Other | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	2.75%	2.75%
Other | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	10.00%	10.00%