QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Quarterly financial data (unaudited)
Transmission revenues	$ 88	$ 85	$ 82	$ 86	$ 87	$ 84	$ 84	$ 88	$ 341	[1]	$ 343	[1]	$ 353	[1]
Equity earnings	19	15	15	18	22	24	24	29	67	[1]	99	[1]	135	[1]
Net income	50	46	42	53	53	57	54	65	191	[1]	229	[1]	250	[1]
Net income attributable to controlling interests	41	37	34	43	44	48	46	54	155	[1]	192	[1]	216	[1]
Net income per common unit (in dollars per unit)	$ 0.63	$ 0.58	$ 0.40	$ 0.52	$ 0.56	$ 0.64	$ 0.60	$ 0.71	$ 2.13	[1]	$ 2.51	[1]	$ 3.02	[1]
Cash distribution paid	$ 52	$ 52	$ 43	$ 43	$ 43	$ 43	$ 42	$ 42	$ 188	[1]	$ 169	[1]	$ 155	[1]

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