SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
		Feb. 15, 2017	Jan. 26, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Subsequent Event [Line Items]
Amount of distribution				$ 813.8	$ 791.4	$ 731.0
Line of Credit Facility, Maximum Borrowing Capacity				$ 750.0
Cash distributions paid per limited partner unit outstanding (in dollars per share)				$ 2.33	$ 2.31	$ 2.20
Cash available for distribution				$ 1,047.0	$ 1,000.4	$ 874.7
Total amount of distribution paid				$ 864.2	$ 835.9	727.8
Master limited partnership ownership interest (in percentage)				100.00%	100.00%
I Units [Member]
Subsequent Event [Line Items]
Amount of distribution	[1]			$ 179.1	$ 161.2	143.9
Subsequent Event [Member]
Subsequent Event [Line Items]
Cash distributions paid per limited partner unit outstanding (in dollars per share)			$ 0.5830
Cash available for distribution			$ 264.8
Distribution payment date			Feb. 14, 2017
Distribution record date			Feb. 07, 2017
Distribution declaration date			Jan. 26, 2017
Total amount of distribution paid			$ 216.1
Noncontrolling Interest, Ownership Percentage by Parent		75.00%
Subsequent Event [Member] | Marathon Petroleum Corporation [Member]
Subsequent Event [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		25.00%
Subsequent Event [Member] | Bakken Pipeline System [Member]
Subsequent Event [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent		75.00%
Business Acquisition, Percentage of Voting Interests Acquired		27.60%
Business Combination, Consideration Transferred		$ 1,500.0
Subsequent Event [Member] | Bakken Pipeline Investments LLC [Member]
Subsequent Event [Line Items]
Equity Method Investment, Ownership Percentage		49.00%
Subsequent Event [Member] | Revolving Credit Facility [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,500.0
Subsequent Event [Member] | I Units [Member]
Subsequent Event [Line Items]
Amount of distribution			47.7
Amount paid to parent			$ 1.0
Master limited partnership ownership interest (in percentage)			2.00%
Series EA Interests [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount paid to parent			$ 68.8
Amount paid to the noncontrolling interest			$ 22.9
Distribution declaration date			Jan. 26, 2017
Series ME Interests [Member]
Subsequent Event [Line Items]
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
Series ME Interests [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount paid to parent			$ 42.7
Amount paid to the noncontrolling interest			$ 14.2
Distribution declaration date			Jan. 26, 2017
Midcoast Energy Partners [Member]
Subsequent Event [Line Items]
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] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount of distribution			$ 7.6
Amount paid to parent			$ 8.9
Cash distributions paid per limited partner unit outstanding (in dollars per share)			$ 0.3575
Cash available for distribution			$ 16.5
Distribution payment date			Feb. 14, 2017
Distribution record date			Feb. 07, 2017
Distribution declaration date			Jan. 26, 2017
Midcoast Operating [Member]
Subsequent Event [Line Items]
Reduction of distribution				$ 15.9
Maximum quarterly distribution per share				$ 0.005
Midcoast Operating [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount paid to parent			$ 7.9
Amount paid to the noncontrolling interest			$ 27.9
Distribution payment date			Feb. 14, 2017
Distribution record date			Feb. 07, 2017
Distribution declaration date			Jan. 26, 2017

[1] We issued 8,571,429, 4,980,552 and 4,562,088 i-units to Enbridge Management, L.L.C., the sole owner of our i-units, during 2016, 2015 and 2014, respectively, in lieu of cash distributions.