QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
Quarterly financial data (unaudited)
Transmission revenues	$ 87	$ 80	$ 82	$ 87	$ 88	$ 85	$ 82	$ 86	$ 336	$ 341	[1]	$ 343	[1]
Equity earnings	22	15	18	33	19	15	15	18	88	67	[1]	99	[1]
Net income	53	39	45	67	50	46	42	53	204	191	[1]	229	[1]
Net income attributable to controlling interests	47	31	37	57	41	37	34	43	172	155	[1]	192	[1]
Net income per common unit (in dollars per unit)	$ 0.71	$ 0.48	$ 0.58	$ 0.90	$ 0.63	$ 0.58	$ 0.40	$ 0.52	$ 2.67	$ 2.13		$ 2.51
Cash distributions paid	$ 54	$ 54	$ 52	$ 52	$ 52	$ 52	$ 43	$ 43

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