RELATED PARTY TRANSACTIONS (Distributions by Affiliates) (Details) - USD ($) $ in Millions				1 Months Ended	9 Months Ended
	Aug. 14, 2017	May 15, 2017	Feb. 14, 2017	Feb. 14, 2017	Sep. 30, 2017
Midcoast Energy Partners [Member] | First Quarter Distribution [Member]
Amount paid to parent				$ 8.9
Amount paid to noncontrolling interest				7.6
Total amount of distribution paid				16.5
General Partner [Member] | First Quarter Distribution [Member]
Amount paid to parent				10.2
Amount paid to noncontrolling interest				30.7
Total amount of distribution paid				40.9
Series EA [Member]
Amount paid to parent					$ 85.4
Amount paid to noncontrolling interest					180.6
Total amount of distribution paid					266.0
Series EA [Member] | Third Quarter Distribution [Member]
Amount paid to parent	$ 33.2
Amount paid to noncontrolling interest	49.8
Total amount of distribution paid	83.0
Series EA [Member] | Second Quarter Distribution [Member]
Amount paid to parent		$ 29.3
Amount paid to noncontrolling interest		62.0
Total amount of distribution paid		91.3
Series EA [Member] | First Quarter Distribution [Member]
Amount paid to parent				22.9
Amount paid to noncontrolling interest				68.8
Total amount of distribution paid				$ 91.7
U.S. Mainline Expansion Interests [Member]
Amount paid to parent					40.7
Amount paid to noncontrolling interest					122.0
Total amount of distribution paid					$ 162.7
U.S. Mainline Expansion Interests [Member] | Third Quarter Distribution [Member]
Amount paid to parent	13.8
Amount paid to noncontrolling interest	41.3
Total amount of distribution paid	$ 55.1
U.S. Mainline Expansion Interests [Member] | Second Quarter Distribution [Member]
Amount paid to parent		12.7
Amount paid to noncontrolling interest		38.0
Total amount of distribution paid		$ 50.7
U.S. Mainline Expansion Interests [Member] | First Quarter Distribution [Member]
Amount paid to parent			$ 14.2
Amount paid to noncontrolling interest			42.7
Total amount of distribution paid			$ 56.9