QUARTERLY FINANCIAL DATA (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly financial data (unaudited)
Transmission revenues		$ 89	$ 83	$ 85	$ 87	$ 87	$ 80	$ 82	$ 87	$ 344	$ 336	$ 341	[1]
Equity earnings		34	17	15	31	22	15	18	33	97	88	67	[1]
Impairment of equity-method investment		(199)								(199)
Net income (loss)		(137)	49	44	64	53	39	45	67	20	204	191	[1]
Net income (loss) attributable to controlling interests		(137)	49	44	57	47	31	37	57	13	172	155	[1]
Cash distributions paid		59	$ 59	$ 55	$ 55	$ 55	$ 54	$ 52	$ 51
Great Lakes
Quarterly financial data (unaudited)
Equity earnings										31	19	3
Great Lakes | Nonrecurring fair value measurement
Quarterly financial data (unaudited)
Impairment of equity-method investment		$ (199)
Northern Border
Quarterly financial data (unaudited)
Equity earnings										$ 66	$ 69	$ 64
Northern Border | Nonrecurring fair value measurement
Quarterly financial data (unaudited)
Impairment of equity-method investment	$ 0
Common units
Quarterly financial data (unaudited)
Net income per common unit (in dollars per unit)		$ (2.24)	$ 0.70	$ 0.66	$ 0.88	$ 0.71	$ 0.48	$ 0.58	$ 0.90	$ (0.03)	$ 2.67	$ 2.13	[1]

[1] Recast as discussed in Note 2 and Note 6.