CONSOLIDATED STATEMENT OF CHANGES IN PARTNERS' EQUITY - USD ($) shares in Millions, $ in Millions		ATM Equity Issuance Program Limited Partners Common Units	ATM Equity Issuance Program General Partner	ATM Equity Issuance Program	[1]	Limited Partners GTN Common Units	Limited Partners Portland Natural Gas Transmission System Common Units	Limited Partners Portland Natural Gas Transmission System And Iroquois Acquisition Common Units	Limited Partners Common Units		Limited Partners Class B Units		General Partner GTN	General Partner Portland Natural Gas Transmission System	General Partner Portland Natural Gas Transmission System And Iroquois Acquisition	General Partner		Accumulated Other Comprehensive Income (Loss)	[2],[3]	GTN Non-Controlling Interest	[1]	GTN	[1]	Portland Natural Gas Transmission System	Portland Natural Gas Transmission System And Iroquois Acquisition Equity of former parent of PNGTS	[1],[2]	Portland Natural Gas Transmission System And Iroquois Acquisition	[1]	Class B Units	[1]	Non-Controlling Interest		Equity of former parent of PNGTS	[2]	Total
Partners' Equity at beginning of year at Dec. 31, 2014	[1]								$ 1,325							$ 29		$ (5)													$ 323		$ 146		$ 1,818
Partners' Equity at beginning of year (in units) at Dec. 31, 2014	[1]								63.6
Increase (Decrease) in Partners' Equity
Net income (loss)									$ (2)	[1]	$ 12	[1]				3	[1]														21	[1]	24	[1]	58	[4]
Net other comprehensive income	[1]																	1																	1
Equity issuance, net		$ 43	$ 1	$ 44							$ 95																		$ 95
Equity Issuance, net (in units)		0.7									1.9
Acquisition of interests (note 7)						$ (124)							$ (3)							$ (232)		$ (359)
Equity Contribution (Note 7)																2																			2	[1]
Distributions	[1]								(221)							(7)															(21)		(19)		(268)
Partners' Equity at end of year at Dec. 31, 2015	[1]								$ 1,021		$ 107					25		(4)													91		151		1,391
Partners' Equity at end of year (in units) at Dec. 31, 2015	[1]								64.3		1.9
Increase (Decrease) in Partners' Equity
Net income (loss)									$ 211	[1]	$ 22	[1]				11	[1]														15	[1]	4	[1]	263	[4]
Net other comprehensive income	[1]																	2													1				3
Equity issuance, net		$ 82	2	84
Equity Issuance, net (in units)		1.5
Common unit issuance subject to rescission, net (Note 10)	[5]								$ 81							2																			83	[1]
Common unit issuance subject to rescission, net (Note 10) (in units)	[5]								1.6
Reclassification of common units no longer subject to rescission (Note 10)	[5]								$ (81)							(2)																			(83)	[1]
Acquisition of interests (note 7)	[1]						$ (72)							$ (1)										$ (73)
Distributions	[1]								(240)		(12)					(10)															(10)		(4)		(276)
Former parent carrying amount of PNGTS	[1]																																(120)		(120)
Partners' Equity at end of year at Dec. 31, 2016	[1]								$ 1,002		$ 117					27		(2)													97		31		1,272
Partners' Equity at end of year (in units) at Dec. 31, 2016	[1]								67.4		1.9
Increase (Decrease) in Partners' Equity
Net income (loss)									$ 219		$ 15					16															11	[1]	2	[1]	263	[1]
Net other comprehensive income																		7																	7	[1]
Equity issuance, net		$ 173	$ 3	$ 176
Equity Issuance, net (in units)		3.2
Reclassification of common units no longer subject to rescission (Note 10)									81							2																			83	[1]
Acquisition of interests (note 7)								$ (383)							$ (8)										$ (32)		$ (423)
Distributions									(268)		(22)					(16)															(3)	[1]	$ (1)	[1]	(310)	[1]
Partners' Equity at end of year at Dec. 31, 2017	[1]								$ 824		$ 110					$ 24		$ 5													$ 105				$ 1,068
Partners' Equity at end of year (in units) at Dec. 31, 2017	[1]								70.6		1.9

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

[2] Equity of Former Parent of PNGTS.

[3] Losses related to cash flow hedges reported in AOCI and expected to be reclassified to net income in the next 12 months are estimated to be $2 million. These estimates assume constant interest rates over time; however, the amounts reclassified will vary based on actual value of interest rates at the date of settlement

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

[5] These units are treated as outstanding for financial reporting purposes.