MARKETABLE SECURITIES - Equity Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Equity securities	$ 40.9	$ 45.7
Amortized Cost	213.1	228.4
Pre-Tax Unrealized Gains	11.2	5.8
Pre-Tax Unrealized Losses	(0.1)	(0.1)
Fair Value	224.2	234.1
CYAPC and YAEC
Debt Securities, Available-for-sale [Line Items]
Equity securities	205.1	182.8
Marketable securities held in nuclear decommissioning trusts	192.5	198.1
Accounting Standards Update 2016-01
Debt Securities, Available-for-sale [Line Items]
Unrealized gains recorded in other income	$ 3.7	$ 9.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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