MARKETABLE SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Available-for-sale equity securities	$ 41.6		$ 41.6		$ 45.7
Unrealized gain (loss) recorded in other income	6.6	$ 2.3	(2.5)	$ 3.3
CYAPC and YAEC
Debt Securities, Available-for-sale [Line Items]
Available-for-sale equity securities	172.3		172.3		182.8
Marketable securities held in nuclear decommissioning trust	$ 193.6		$ 193.6		$ 198.1