NONCONTROLLING INTERESTS (Components of net income (loss) attributable to noncontrolling interests) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest		$ 79.2	$ (191.9)	$ 70.3	$ 68.8	$ 82.0	$ 77.8	$ 10.0	$ 51.3	$ 26.4	$ 221.1	$ 263.3
Alberta Clipper Interests [Member]
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest										0.0	(0.8)	54.6
Eastern Access Interest [Member]
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest										204.7	189.3	145.3
US Mainline Expansion Interests [Member]
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest										138.5	108.2	33.8
North Dakota Pipeline Company Interests [Member]
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest	[1]									(270.8)	0.0	0.0
Midcoast Energy Partners, L.P. [Member]
Noncontrolling Interest [Line Items]
Net income (loss) attributable to Noncontrolling interest										$ (46.0)	$ (75.6)	$ 29.6

[1] Inclusive of impairment on the Sandpiper Project attributable to noncontrolling interest for the year ending December 31, 2016. For further details, see Note 10. Property, Plant and Equipment.