MARKETABLE SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale equity securities	$ 45.2		$ 45.2		$ 44.0
Unrealized gains recorded in other income	2.3	$ 0.9	3.3	$ 0.2
CYAPC and YAEC
Debt Securities, Available-for-sale [Line Items]
Available-for-sale equity securities	158.5		158.5		200.0
Marketable securities held in nuclear decommissioning trust	$ 172.4		$ 172.4		$ 143.9