DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES (Effect of Derivative Instruments on Consolidated Statements of Income) (Details) - Derivatives Not Designated As Hedging Instruments [Member] - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]	$ 41.7	$ 24.1	$ 29.7	$ 1.2
Transportation and Other Services [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2],[3]	8.1	7.0	8.1	(0.6)
Gain (Loss) on Settlement of Derivative Instruments		7.0	0.6	19.2	0.9
Commodity Sales [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]	$ (7.2)	$ 7.6	(22.4)	10.7
Commodity Sales - Affiliate [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]			(0.3)	0.5
Commodity Costs [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2],[4]	$ 40.8	$ 9.5	44.3	(9.9)
Gain (Loss) on Settlement of Derivative Instruments		$ 27.3	$ 0.2	$ 71.0	(8.6)
Power [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]				$ 0.5

[1] Does not include settlements associated with derivative instruments that settle through physical delivery.

[2] Includes only net gains or losses associated with those derivatives that do not qualify for hedge accounting treatment and does not include the ineffective portion of derivatives that are designated as hedging instruments.

[3] Includes settlement gains of $7.0 million and $0.6 million for the three months ended September 30, 2015 and 2014, respectively, and settlement gains of $19.2 million and $0.9 million for the nine months ended September 30, 2015 and 2014, respectively.

[4] Includes settlement gains of $27.3 million and $0.2 million for the three months ended September 30, 2015 and 2014, respectively, and settlement gains (losses) of $71.0 million and $(8.6) million for the nine months ended September 30, 2015 and 2014, respectively.