MARKETABLE SECURITIES - Schedule of Available-for-Sale Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable securities held in nuclear decommissioning plant	$ 503.6	$ 466.7
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	284.9	296.2
Pre-Tax Unrealized Gains	3.2	1.1
Pre-Tax Unrealized Losses	(1.1)	(2.1)
Fair Value	287.0	295.2
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	216.1	203.3
Pre-Tax Unrealized Gains	97.8	62.3
Pre-Tax Unrealized Losses	(0.1)	(1.2)
Fair Value	313.8	264.4
Fair Value, Inputs, Level 1
Schedule of Available-for-sale Securities [Line Items]
Net gains on fair value option		$ 0.6	$ 2.0
Other Income | Fair Value, Inputs, Level 1
Schedule of Available-for-sale Securities [Line Items]
Net gains on fair value option	$ 9.8