SUBSEQUENT EVENTS (Details) $ in Thousands	3 Months Ended	12 Months Ended			36 Months Ended	132 Months Ended	156 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) item MW	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2022 USD ($) employee	Dec. 31, 2030 USD ($) employee	Dec. 31, 2032 USD ($) employee
Subsequent event
Impairment loss			$ 0	$ 0
Net book value		$ 3,613,846	$ 3,607,484
Colowyo Mine
Subsequent event
Net book value		$ 239,000
Escalante Generating Station
Subsequent event
Generating unit capacity (in megawatts) | MW		253
Craig Generating Station
Subsequent event
Number of units in the facility | item		3
Craig Generating Station, Unit 2
Subsequent event
Generating unit capacity (in megawatts) | MW		410
Net book value		$ 82,500
Craig Generating Station, Unit 3
Subsequent event
Generating unit capacity (in megawatts) | MW		448
Net book value		$ 348,600
Subsequent Event | Colowyo Mine | Forecast
Subsequent event
Increase in annual depreciation expense						$ 12,700
Number of employees impacted | employee						219
Subsequent Event | Escalante Generating Station | Forecast
Subsequent event
Impairment loss	$ 282,000
Decommission, employee related, and other expenses					$ 26,000
Number of employees impacted | employee					107
Subsequent Event | Craig Generating Station, Units 2 and 3 | Forecast
Subsequent event
Decommission, employee related, and other expenses							$ 40,000
Number of employees impacted | employee							253
Subsequent Event | Craig Generating Station, Unit 2 | Forecast
Subsequent event
Increase in annual depreciation expense						$ 6,600
Subsequent Event | Craig Generating Station, Unit 3
Subsequent event
Increase in annual depreciation expense						$ 21,100