PARTNERS' CAPITAL (Schedule of Changes in Partners' Capital) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Beginning balance			$ 5,120.4
Net income	$ 82.1	$ 20.5	125.1	$ 157.7
Accretion of discount on preferred units	2.1	3.8	10.1	11.1
Ending balance	5,156.9		5,156.9
Noncontrolling interest:
Noncontrolling interest, beginning balance			3,609.0	1,975.6
Capital contributions			$ 740.6	1,083.0
Transfer of interests in subsidiary to Midcoast Energy Partners, L.P.				$ 125.4
Acquisition of noncontrolling interest in subsidiary			$ (403.7)
Other comprehensive income (loss) allocated to noncontrolling interest	(1.3)	2.1	(3.9)	$ 1.8
Net income	77.8	70.7	139.1	149.4
Distributions to noncontrolling interest			(178.7)	(80.9)
Noncontrolling interest, ending balance	3,902.4	3,254.3	3,902.4	3,254.3
Total partners' capital	9,059.3	8,392.9	9,059.3	8,392.9	$ 8,729.4
Series 1 Preferred Interests [Member]
Beginning balance			1,175.6	1,160.7
Net income			67.5	67.5
Distributions			(67.5)	(67.5)
Accretion of discount on preferred units			10.1	11.1
Ending balance	1,185.7	1,171.8	1,185.7	1,171.8
General and Limited Partner Interests [Member]
Beginning balance			4,156.2	$ 4,637.7
Proceeds from issuance of partnership interests, net of costs			294.8
Net income			125.1	$ 157.7
Distributions			$ (619.9)	(544.2)
Transfer of interests in subsidiary to Midcoast Energy Partners, L.P.				$ (125.4)
Acquisition of noncontrolling interest in subsidiary			$ 403.7
Ending balance	4,359.9	4,125.8	4,359.9	$ 4,125.8
Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance			(211.4)	(76.6)
Changes in fair value of derivative financial instruments reclassified to earnings			(6.4)	22.2
Changes in fair value of derivative financial instruments recognized in other comprehensive loss			(170.9)	(104.6)
Ending balance	$ (388.7)	$ (159.0)	$ (388.7)	$ (159.0)