RELATED PARTY TRANSACTIONS (Distributions by Affiliates) (Details) - USD ($) $ in Millions			1 Months Ended											12 Months Ended
	Feb. 12, 2016	Nov. 13, 2015	Nov. 14, 2016	Nov. 14, 2016	Aug. 12, 2016	May 13, 2016	Aug. 14, 2015	May 15, 2015	Feb. 13, 2015	Nov. 14, 2014	Aug. 14, 2014	May 15, 2014	Feb. 14, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total amount of distribution paid														$ 864.2	$ 835.9	$ 727.8
First Quarter Distribution [Member]
Distribution declaration date														Jan. 29, 2016	Jan. 29, 2015	Jan. 30, 2014
Distribution payment date														Feb. 12, 2016	Feb. 13, 2015	Feb. 14, 2014
Total amount of distribution paid	$ 216.0								$ 194.2				$ 178.4
Second Quarter Distribution [Member]
Distribution declaration date														Apr. 29, 2016	Apr. 30, 2015	Apr. 30, 2014
Distribution payment date														May 13, 2016	May 15, 2015	May 15, 2014
Total amount of distribution paid						$ 216.0		$ 209.6				$ 178.5
Third Quarter Distribution [Member]
Distribution declaration date														Jul. 28, 2016	Jul. 30, 2015	Jul. 31, 2014
Distribution payment date														Aug. 12, 2016	Aug. 14, 2015	Aug. 14, 2014
Total amount of distribution paid					$ 216.1		$ 216.1				$ 187.2
Fourth Quarter Distribution [Member]
Distribution declaration date														Oct. 28, 2016	Oct. 30, 2015	Oct. 31, 2014
Distribution payment date														Nov. 14, 2016	Nov. 13, 2015	Nov. 14, 2014
Total amount of distribution paid		$ 216.0		$ 216.1						$ 183.7
Midcoast Energy Partners [Member]
Amount paid to parent														$ 35.6	$ 34.8	$ 28.5
Amount paid to the noncontrolling interest														30.4	29.8	24.2
Total amount of distribution paid														$ 66.0	$ 64.6	$ 52.7
Midcoast Energy Partners [Member] | First Quarter Distribution [Member]
Distribution declaration date														Jan. 28, 2016	Jan. 28, 2015	Jan. 29, 2014
Distribution payment date														Feb. 12, 2016	Feb. 13, 2015	Feb. 14, 2014
Amount paid to parent														$ 8.9	$ 8.5	$ 4.2
Amount paid to the noncontrolling interest														7.6	7.3	3.5
Total amount of distribution paid														$ 16.5	$ 15.8	$ 7.7
Midcoast Energy Partners [Member] | Second Quarter Distribution [Member]
Distribution declaration date														Apr. 28, 2016	Apr. 29, 2015	Apr. 29, 2014
Distribution payment date														May 13, 2016	May 15, 2015	May 15, 2014
Amount paid to parent														$ 8.9	$ 8.6	$ 7.8
Amount paid to the noncontrolling interest														7.6	7.4	6.6
Total amount of distribution paid														$ 16.5	$ 16.0	$ 14.4
Midcoast Energy Partners [Member] | Third Quarter Distribution [Member]
Distribution declaration date														Jul. 27, 2016	Jul. 29, 2015	Jul. 31, 2014
Distribution payment date														Aug. 12, 2016	Aug. 14, 2015	Aug. 14, 2014
Amount paid to parent														$ 8.9	$ 8.8	$ 8.1
Amount paid to the noncontrolling interest														7.6	7.5	6.9
Total amount of distribution paid														$ 16.5	$ 16.3	$ 15.0
Midcoast Energy Partners [Member] | Fourth Quarter Distribution [Member]
Distribution declaration date														Oct. 27, 2016	Oct. 29, 2015	Oct. 30, 2014
Distribution payment date														Nov. 14, 2016	Nov. 13, 2015	Nov. 14, 2014
Amount paid to parent														$ 8.9	$ 8.9	$ 8.4
Amount paid to the noncontrolling interest														7.6	7.6	7.2
Total amount of distribution paid														16.5	$ 16.5	15.6
Alberta Clipper Pipeline [Member]
Amount paid to parent																30.5
Amount paid to the noncontrolling interest																61.0
Total amount of distribution paid																$ 91.5
Alberta Clipper Pipeline [Member] | First Quarter Distribution [Member]
Distribution declaration date															Jan. 29, 2015	Jan. 30, 2014
Distribution payment date															Feb. 13, 2015	Feb. 14, 2014
Amount paid to parent									13.7				6.4
Amount paid to the noncontrolling interest									27.5				12.8
Total amount of distribution paid									41.2				$ 19.2
Alberta Clipper Pipeline [Member] | Second Quarter Distribution [Member]
Distribution declaration date																Apr. 30, 2014
Distribution payment date																May 15, 2014
Amount paid to parent												6.6
Amount paid to the noncontrolling interest												13.1
Total amount of distribution paid												19.7
Alberta Clipper Pipeline [Member] | Third Quarter Distribution [Member]
Distribution declaration date																Jul. 31, 2014
Distribution payment date																Aug. 14, 2014
Amount paid to parent											7.4
Amount paid to the noncontrolling interest											14.8
Total amount of distribution paid											22.2
Alberta Clipper Pipeline [Member] | Fourth Quarter Distribution [Member]
Distribution declaration date																Oct. 31, 2014
Distribution payment date																Nov. 14, 2014
Amount paid to parent										10.1
Amount paid to the noncontrolling interest										20.3
Total amount of distribution paid										30.4
Series EA [Member]
Amount paid to parent														200.8	$ 191.3	$ 22.7
Amount paid to the noncontrolling interest														127.7	119.3	66.9
Total amount of distribution paid														$ 328.5	$ 310.6	$ 89.6
Series EA [Member] | First Quarter Distribution [Member]
Distribution declaration date														Jan. 29, 2016	Jan. 29, 2015
Distribution payment date														Feb. 12, 2016	Feb. 13, 2015
Amount paid to parent	79.2								22.3
Amount paid to the noncontrolling interest	0.0								67.0
Total amount of distribution paid	$ 79.2								$ 89.3
Series EA [Member] | Second Quarter Distribution [Member]
Distribution declaration date														Apr. 29, 2016	Apr. 30, 2015	Apr. 29, 2014
Distribution payment date														May 13, 2016	May 15, 2015	May 15, 2014
Amount paid to parent						79.0		17.5				2.5
Amount paid to the noncontrolling interest						0.0		52.3				6.5
Total amount of distribution paid						$ 79.0		$ 69.8				$ 9.0
Series EA [Member] | Third Quarter Distribution [Member]
Distribution declaration date														Jul. 28, 2016	Jul. 30, 2015	Jul. 31, 2014
Distribution payment date														Aug. 12, 2016	Aug. 14, 2015	Aug. 14, 2014
Amount paid to parent					21.0		75.4				5.6
Amount paid to the noncontrolling interest					63.0		0.0				16.7
Total amount of distribution paid					$ 84.0		$ 75.4				$ 22.3
Series EA [Member] | Fourth Quarter Distribution [Member]
Distribution declaration date														Oct. 28, 2016	Oct. 30, 2015	Oct. 31, 2014
Distribution payment date														Nov. 14, 2016	Nov. 13, 2015	Nov. 14, 2014
Amount paid to parent		76.1	$ 21.6							14.6
Amount paid to the noncontrolling interest		0.0	64.7							43.7
Total amount of distribution paid		$ 76.1	$ 86.3							$ 58.3
U.S. Mainline Expansion Interests [Member]
Amount paid to parent														$ 111.9	$ 55.5
Amount paid to the noncontrolling interest														83.7	9.7
Total amount of distribution paid														$ 195.6	$ 65.2
U.S. Mainline Expansion Interests [Member] | First Quarter Distribution [Member]
Distribution declaration date														Jan. 29, 2016	Jan. 29, 2015
Distribution payment date														Feb. 12, 2016	Feb. 13, 2015
Amount paid to parent														$ 40.8	$ 1.8
Amount paid to the noncontrolling interest														0.0	5.2
Total amount of distribution paid														$ 40.8	$ 7.0
U.S. Mainline Expansion Interests [Member] | Second Quarter Distribution [Member]
Distribution declaration date														Apr. 29, 2016	Apr. 30, 2015
Distribution payment date														May 13, 2016	May 15, 2015
Amount paid to parent														$ 43.2	$ 1.5
Amount paid to the noncontrolling interest														0.0	4.5
Total amount of distribution paid														$ 43.2	$ 6.0
U.S. Mainline Expansion Interests [Member] | Third Quarter Distribution [Member]
Distribution declaration date														Jul. 28, 2016	Jul. 30, 2015
Distribution payment date														Aug. 12, 2016	Aug. 14, 2015
Amount paid to parent														$ 13.1	$ 19.7
Amount paid to the noncontrolling interest														39.4	0.0
Total amount of distribution paid														$ 52.5	$ 19.7
U.S. Mainline Expansion Interests [Member] | Fourth Quarter Distribution [Member]
Distribution declaration date														Oct. 28, 2016	Oct. 30, 2015	Oct. 31, 2014
Distribution payment date														Nov. 14, 2016	Nov. 13, 2015	Nov. 14, 2014
Amount paid to parent														$ 14.8	$ 32.5	$ 0.6
Amount paid to the noncontrolling interest														44.3	0.0	1.9
Total amount of distribution paid														$ 59.1	$ 32.5	$ 2.5