SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended		6 Months Ended
	Aug. 14, 2015	Jul. 30, 2015	Jul. 29, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Subsequent Event [Line Items]
Amount of Distribution				$ 198.5	$ 182.2	$ 392.0	$ 359.9
Subsequent Event [Member]
Subsequent Event [Line Items]
Distribution declaration date						Jul. 30, 2015
Record Date						Aug. 07, 2015
Distribution payment date						Aug. 14, 2015
Cash available for distribution				$ 257.9		$ 257.9
Cash distributions paid per limited partner unit outstanding (in dollars per share)	$ 0.5830
Total amount of distribution paid	$ 216.1
Maximum reduction in future distributions (in percentage)		50.00%
Series EA Interests [Member]
Subsequent Event [Line Items]
Total amount of distribution paid						159.1
Amount Paid To Parent						39.8
Amount paid to the noncontrolling interest						$ 119.3
Noncontrolling interest, ownership by parent (in percentage)				75.00%		75.00%
Ownership interest in affiliate of Partnership (in percentage)						25.00%
Series EA Interests [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount Paid To Parent		$ 56.5
Noncontrolling interest, ownership by parent (in percentage)				75.00%		75.00%
Ownership interest in affiliate of Partnership (in percentage)						25.00%
Series ME Interests [Member]
Subsequent Event [Line Items]
Total amount of distribution paid						$ 13.0
Amount Paid To Parent						3.3
Amount paid to the noncontrolling interest						$ 9.7
Noncontrolling interest, ownership by parent (in percentage)				75.00%		75.00%
Ownership interest in affiliate of Partnership (in percentage)						25.00%
Series ME Interests [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount Paid To Parent		$ 14.8
Noncontrolling interest, ownership by parent (in percentage)				75.00%		75.00%
Ownership interest in affiliate of Partnership (in percentage)						25.00%
Midcoast Energy Partners [Member]
Subsequent Event [Line Items]
Total amount of distribution paid						$ 31.8
Amount Paid To Parent						17.1
Amount paid to the noncontrolling interest						$ 14.7
Midcoast Energy Partners [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Distribution declaration date						Jul. 29, 2015
Record Date						Aug. 07, 2015
Distribution payment date						Aug. 14, 2015
Cash available for distribution				$ 16.3		$ 16.3
Cash distributions paid per limited partner unit outstanding (in dollars per share)	$ 0.3525
Amount of Distribution	$ 7.5
Amount Paid To Parent	8.8
Midcoast Operating [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount Paid To Parent	27.9
Amount paid to the noncontrolling interest	26.2
Maximum quarterly distribution (dollars per share)			$ 0.005
i-Units [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount of Distribution	41.0
General Partner [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount Paid To Parent	$ 0.8