MARKETABLE SECURITIES - Available for Sale Securities Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Debt, equity and marketable securities held in nuclear decommissioning trust	$ 485.9	$ 466.7

- Definition

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.

+ References + Details