EQUITY INVESTMENTS (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended				6 Months Ended
	Aug. 01, 2017	Jun. 01, 2017	Apr. 30, 2006	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016		Jun. 30, 2017	Jun. 30, 2016		Jul. 31, 2017	Dec. 31, 2016
EQUITY INVESTMENTS
Equity Earnings				$ 24		$ 20	[1]	$ 60	$ 53	[1]
Equity Investments				1,138		918		1,138	918			$ 918	[1]
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt				$ 57				57				52	[1]
Amount borrowed								607	205	[1]
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment								$ 9
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)				50.00%				50.00%			50.00%
Equity Earnings				$ 15		16		$ 34	34
Equity Investments				437		444		437	444
Amortization period of transaction fee			12 years
Transaction fee			$ 10
Additional ownership interest acquired (as a percent)			20.00%
Assets
Cash and cash equivalents				16				16				14
Other current assets				34				34				36
Plant, property and equipment, net				1,077				1,077				1,089
Other assets				14				14				14
Assets, total				1,141				1,141				1,153
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				37				37				38
Deferred credits and other				30				30				28
Long-term debt, including current maturities, net				430				430				430
Partners' equity
Partners' equity				645				645				659
Accumulated other comprehensive loss				(1)				(1)				(2)
Liabilities and Partners' Equity, total				1,141				1,141				1,153
Revenues (expenses)
Transmission revenues				69		70		144	144
Operating expenses				(18)		(18)		(36)	(35)
Depreciation				(15)		(15)		(30)	(29)
Financial charges and other				(5)		(6)		(9)	(11)
Net income				$ 31		31		$ 69	69
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)	46.45%			46.45%				46.45%
Equity Earnings				$ 6		4		$ 23	19
Equity Investments				475		474		475	474
Equity contribution					$ 4			4	4	[1]
Assets
Current assets				61				61				66
Plant, property and equipment, net				705				705				714
Assets, total				766				766				780
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				34				34				40
Long-term debt, including current maturities, net				269				269				278
Partners' equity
Partners' equity				463				463				462
Liabilities and Partners' Equity, total				766				766				780
Revenues (expenses)
Transmission revenues				41		36		103	97
Operating expenses				(17)		(15)		(30)	(30)
Depreciation				(7)		(7)		(14)	(14)
Financial charges and other				(5)		(5)		(10)	(11)
Net income				$ 12		9		$ 49	42
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)		49.34%		49.34%				49.34%
Equity Earnings				$ 3				$ 3
Equity Investments				226				226
Undistributed earnings								0
Equity contribution	$ 28	$ 710
Assets
Cash and cash equivalents				100				100				88
Other current assets				26				26				27
Plant, property and equipment, net				595				595				611
Other assets				8				8				7
Assets, total				729				729				733
LIABILITIES AND PARTNERS' EQUITY
Current liabilities				15				15				16
Long-term debt, including current maturities, net				332				332				338
Other non-current liabilities				9				9				5
Partners' equity
Partners' equity				373				373				374
Liabilities and Partners' Equity, total				729				729				$ 733
Revenues (expenses)
Transmission revenues				45		47		98	100
Operating expenses				(13)		(15)		(29)	(30)
Depreciation				(7)		(10)		(14)	(19)
Financial charges and other				(5)		(4)		(9)	(8)
Net income				$ 20		$ 18		$ 46	$ 43

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).