CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Cash Generated From Operations
Net income		$ 263	[1]	$ 263	[2]	$ 58	[2]
Depreciation		97		96	[3]	95	[3]
Impairment of equity-method investment (Note 5)	[3]					199
Amortization of debt issue costs reported as interest expense (Note 12)		2		2	[3]	1	[3]
Amortization of realized loss on derivative instrument (Note 19)		1		1	[3]	1	[3]
Accrual of costs related to acquisition of 49.9% interest in PNGTS (Note 7)	[3]					2
Equity earnings from equity investments (Note 5)		(124)		(97)	[3]	(97)	[3]
Distributions received from operating activities of equity investments (Note 5)		140		153	[3]	119	[3]
Provision for deferred state income taxes (Note 24)	[3]					4
Provision for rate refund-PNGTS (Note 2)	[3]					(101)
Equity allowance for funds used during construction		(1)				(1)	[3]
Change in operating working capital (Note 15)		(2)		(1)	[3]	(20)	[3]
Net Cash Provided by (Used in) Operating Activities		376		417	[3]	260	[3]
Investing Activities
Distribution received as return of investment (Note 5)		5		0		0
Capital expenditures		(29)		(29)	[3]	(54)	[3]
Other		1		1	[3]	1	[3]
Net Cash Provided by (Used in) Investing Activities		(761)		(230)	[3]	(326)	[3]
Financing Activities
Distributions paid (Note 14)		(284)		(250)	[3]	(228)	[3]
Distributions paid to non-controlling interests		(5)		(12)	[3]	(21)	[3]
Common unit issuance, net (Note 10)		176		84	[3]	44	[3]
Common unit issuance subject to rescission, net (Note 10)	[3]			83
Equity contribution by the General Partner (Note 7)	[3]					2
Long-term debt issued, net of discount (Note 8)		802		209	[3]	618	[3]
Long-term debt repaid (Note 8)		(310)		(270)	[3]	(425)	[3]
Debt issuance costs		(2)		(1)	[3]	(3)	[3]
Total financing activities		354		(178)	[3]	(32)	[3]
Increase/(decrease) in cash and cash equivalents		(31)		9	[3]	(98)	[3]
Cash and cash equivalents, beginning of year	[3]	64		55		153
Cash and cash equivalents, end of year		33		64	[3]	55	[3]
Interest payments paid		79		66	[3]	59	[3]
State income taxes paid		2		2	[3]	2	[3]
Supplemental information about non-cash investing and financing activities
Accrued capital expenditures		9				10	[3]
Issuance of Class B units to TransCanada (Note 10)	[3]					95
Class B Units
Financing Activities
Distributions paid (Note 10 and 14)		(22)		(12)	[3]
Northern Border
Cash Generated From Operations
Equity earnings from equity investments (Note 5)		(67)		(69)		(66)
Investing Activities
Investment/Acquisition of interests		(83)
Great Lakes
Cash Generated From Operations
Equity earnings from equity investments (Note 5)		(31)		(28)		(31)
Provision for rate refund-PNGTS (Note 2)		8
Investing Activities
Investment/Acquisition of interests		(9)		(9)	[3]	(9)	[3]
Portland Natural Gas Transmission System
Investing Activities
Investment/Acquisition of interests	[3]			(193)
Financing Activities
Distributions paid to former parent of PNGTS		(1)		$ (9)	[3]	(19)	[3]
GTN
Investing Activities
Investment/Acquisition of interests	[3]					$ (264)
Iroquois
Cash Generated From Operations
Equity earnings from equity investments (Note 5)		(26)
Investing Activities
Distribution received as return of investment (Note 5)		5
Portland Natural Gas Transmission System And Iroquois Acquisition
Investing Activities
Investment/Acquisition of interests		$ (646)

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[3] Recast to consolidate PNGTS (Refer to Notes 2 and 7).