MARKETABLE SECURITIES - Equity Securities (Details) - USD ($) $ in Millions	1 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Payment to DOE	$ 145.0
Equity securities	44.0
Available-for-sale Securities, Equity Securities		$ 52.5
Amortized Cost	190.0	284.9
Pre-Tax Unrealized Gains	0.4	3.2
Pre-Tax Unrealized Losses	(4.0)	(1.1)
Fair Value	186.4	287.0
Equity Securities \| YAEC
Debt Securities, Available-for-sale [Line Items]
Marketable securities held in nuclear decommissioning trusts	200.0	261.3
Debt Securities \| YAEC
Debt Securities, Available-for-sale [Line Items]
Marketable securities held in nuclear decommissioning trusts	$ 143.9	$ 242.3

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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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