MARKETABLE SECURITIES - Securities Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Marketable securities held in nuclear decommissioning trust	$ 235.7	$ 242.3
Equity securities held in nuclear decommissioning trust	267.3	$ 263.8
Accounting Standards Update 2016-01
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale equity securities	50.0
Unrealized losses recorded in other income	$ 0.7

Investments in debt and equity securities which are categorized neither as trading securities nor held-to-maturity securities, which are intended be sold or mature greater than one year from the balance sheet date or the normal operating cycle, whichever is longer, and that are pledged to one or more secured parties who have the right to buy, sell, or re-pledge the collateral.

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale which are intended to be held for greater than one year or the normal operating cycle, whichever is longer and that are pledged to one or more secured parties who have the right to buy, sell, or re-pledge the collateral.

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