RELATED PARTY TRANSACTIONS (Distributions by Affiliates) (Details) - USD ($) $ in Millions	1 Months Ended		6 Months Ended
	May 15, 2017	Feb. 14, 2017	Jun. 30, 2017
Midcoast Energy Partners [Member] | First Quarter Distribution [Member]
Distribution declaration date			Jan. 26, 2017
Distribution payment date			Feb. 14, 2017
Amount paid to parent		$ 8.9
Amount paid to noncontrolling interest		7.6
Total amount of distribution paid		16.5
Series EA [Member]
Amount paid to parent			$ 52.2
Amount paid to noncontrolling interest			130.8
Total amount of distribution paid			$ 183.0
Series EA [Member] | First Quarter Distribution [Member]
Distribution declaration date			Jan. 26, 2017
Distribution payment date			Feb. 14, 2017
Amount paid to parent		22.9
Amount paid to noncontrolling interest		68.8
Total amount of distribution paid		91.7
Series EA [Member] | Second Quarter Distribution [Member]
Distribution declaration date			Apr. 27, 2017
Distribution payment date			May 15, 2017
Amount paid to parent	$ 29.3
Amount paid to noncontrolling interest	62.0
Total amount of distribution paid	91.3
U.S. Mainline Expansion Interests [Member]
Amount paid to parent			$ 26.9
Amount paid to noncontrolling interest			80.7
Total amount of distribution paid			$ 107.6
U.S. Mainline Expansion Interests [Member] | First Quarter Distribution [Member]
Distribution declaration date			Jan. 26, 2017
Distribution payment date			Feb. 14, 2017
Amount paid to parent		14.2
Amount paid to noncontrolling interest		42.7
Total amount of distribution paid		$ 56.9
U.S. Mainline Expansion Interests [Member] | Second Quarter Distribution [Member]
Distribution declaration date			Apr. 27, 2017
Distribution payment date			May 15, 2017
Amount paid to parent	12.7
Amount paid to noncontrolling interest	38.0
Total amount of distribution paid	$ 50.7