SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Segments and Impairment Losses (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Impairment of long-lived assets
Impairment loss		$ 0.0		$ 0.0
SEGMENT REPORTING:
Number of operating segments | segment	1
Number of reportable segments | segment	1
Holcomb Expansion
Impairment of long-lived assets
Impairment loss			$ 93.5
Nucla Generating Station
Impairment of long-lived assets
Impairment loss	$ 37.1