DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES (Effect of Derivative Instruments on Consolidated Statements of Income) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Transportation And Other Services Revenue [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain on Settlement of Derivative Instruments		$ 5.6	$ 26.9		$ 4.5
Commodity Costs [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain on Settlement of Derivative Instruments		70.7	96.3		8.0
Power [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]	0.0	0.0		0.7
Derivatives Not Designated As Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2],[3]	(44.5)	53.5		178.9
Derivatives Not Designated As Hedging Instruments [Member] | Transportation And Other Services Revenue [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings		(3.1)	11.4	[1],[2],[4]	17.4	[1],[2],[4]
Derivatives Not Designated As Hedging Instruments [Member] | Commodity Sales [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]	0.3	(23.3)		23.7
Derivatives Not Designated As Hedging Instruments [Member] | Commodity Sales - Affiliate [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2]	0.0	(0.3)		0.3
Derivatives Not Designated As Hedging Instruments [Member] | Commodity Costs [Member] | Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Earnings	[1],[2],[5]	$ (41.7)	$ 65.7		$ 136.8

[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 receive hedge accounting treatment and does not include the ineffective portion of derivatives that are designated as hedging instruments.

[3] Includes settlement gains of $70.3 million and $96.3 million and $8.0 million for the years ended December 31, 2016, 2015, and 2014, respectively.

[4] Includes settlement gains of $5.6 million, $26.9 million, and $4.5 million for the years ended December 31, 2016, 2015 and 2014, respectively.

[5] Includes settlement gains of $70.7 million and $96.3 million and $8.0 million for the years ended December 31, 2016, 2015, and 2014, respectively.