RELATED PARTY TRANSACTIONS (Joint Funding Arrangement Discussions) (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
Jan. 26, 2017	Jun. 30, 2017	Jun. 30, 2016
Proceeds from noncontrolling interests		$ 1,285.0	$ 81.9
Acquisition of noncontrolling interest in subsidiary		360.0
Line 3 Replacement [Member]
Acquisition of noncontrolling interest in subsidiary		39.1
Noncontrolling Interest, Increase from Sale of Parent Equity Interest	$ 411.0
Joint Funding Arrangement, Parent, Percentage	99.00%
Partners Capital Account, Contributions	$ 450.1
Option to increase general partnership interest percentage (in percentage)	40.00%
Contributions to noncontrolling interest		$ 100.0
Joint funding arrangement noncontrolling interest (in percentage)	1.00%
Eastern Access Pipeline [Member]
Additional Acquisition Percentages	15.00%
Acquisition of noncontrolling interest in subsidiary	$ 360.0
Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest	25.00%	40.00%
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	75.00%	60.00%
Contributions to noncontrolling interest		$ 5.6	7.2
US Mainline Expansion Interests [Member]
Noncontrolling Interest, Ownership Percentage by Parent		75.00%
Ownership interest in affiliate of Partnership (in percentage)		25.00%
Proceeds from noncontrolling interests		$ 26.0	$ 42.8
Additional Acquisition Percentages		15.00%

The percentage of additional interest acquired by the entity.

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Amount of contributions to non-controlling interest.

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The percentage of joint funding arrangement non-controlling interest.

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Represents the percentage of parent entity's funding in a joint funding arrangement

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The percentage of option to increase general partnership interest percentage.

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The percentage of ownership interest in affiliate of Partnership.

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Represents the purchase by the parent of an additional equity interest in a subsidiary during the period, thereby effecting a change in total (consolidated) equity attributable to the parent. The purchase of the additional equity interest represented by this element increases the parent's controlling interest in the subsidiary.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
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Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of increase in noncontrolling interest from sale of a portion of the parent's controlling interest.

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-Name Accounting Standards Codification
-Topic 810
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Total contributions made by each class of partners (i.e., general, limited and preferred partners).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of cash inflow from a noncontrolling interest. Includes, but is not limited to, purchase of additional shares or other increase in noncontrolling interest ownership.

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