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 PNGTS 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 PNGTS	General Partner Portland Natural Gas Transmission System And Iroquois Acquisition	General Partner	Accumulated Other Comprehensive Income (Loss)	[2],[3]	PNGTS	[1]	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]	Non-controlling interest		Equity of former parent of PNGTS	[2]	Total
Partners' Equity at beginning of year at Dec. 31, 2015	[1]							$ 1,021	$ 107			$ 25	$ (4)								$ 91		$ 151		$ 1,391
Partners' Equity at beginning of year (in units) at Dec. 31, 2015	[1]							64.3	1.9
Increase (Decrease) in Partners' Equity
Net income	[1]							$ 211	$ 22			11									15		4		263
Other comprehensive income	[1]												2								1				3
ATM equity issuances, net (Note 11)		$ 82	$ 2	$ 84
ATM equity issuances, net (Note 11) (in units)		1.5
Common unit issuance subject to rescission, net (Note 11)	[4]							$ 81				2													83	[1]
Common unit issuance subject to rescission, net (Note 11) (in units)	[4]							1.6
Reclassification of common unit issuance subject to rescission, net (Note 11)	[4]							$ (81)				(2)													(83)	[1]
Acquisition of interests (note 8)						$ (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								$ 219	$ 15			16									11	[1]	2	[1]	263	[1]
Other comprehensive income													7												7	[1]
ATM equity issuances, net (Note 11)		$ 173	3	176
ATM equity issuances, net (Note 11) (in units)		3.2
Reclassification of common units no longer subject to rescission (Note 11)								81				2													83	[1]
Acquisition of interests (note 8)							$ (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
Increase (Decrease) in Partners' Equity
Net income								$ (191)	$ 13			(4)									17	[1]			(165)	[1]
Other comprehensive income													3												3	[1]
ATM equity issuances, net (Note 11)		$ 39	$ 1	$ 40
ATM equity issuances, net (Note 11) (in units)		0.7
Distributions								(210)	(15)			(8)									(14)	[1]			(247)	[1]
Partners' Equity at end of year at Dec. 31, 2018								$ 462	$ 108			$ 13	$ 8								$ 108	[1]			$ 699	[1]
Partners' Equity at end of year (in units) at Dec. 31, 2018								71.3	1.9

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 8).

[2] Equity of Former Parent of PNGTS.

[3] Gains or (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] These units are treated as outstanding for financial reporting purposes.