MARKETABLE SECURITIES - Equity Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Equity securities	$ 45.7	$ 44.0
Amortized Cost	228.4	190.0
Pre-Tax Unrealized Gains	5.8	0.4
Pre-Tax Unrealized Losses	(0.1)	(4.0)
Fair Value	234.1	186.4
CYAPC and YAEC
Debt Securities, Available-for-sale [Line Items]
Equity securities	182.8	200.0
Marketable securities held in nuclear decommissioning trusts	198.1	143.9
Accounting Standards Update 2016-01
Debt Securities, Available-for-sale [Line Items]
Unrealized gains recorded in other income	$ 9.8
Unrealized losses recorded in other income		$ 4.3

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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