QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		May 31, 2011 GTN	May 31, 2011 Bison
Quarterly financial data (unaudited)
Equity earnings	$ 29	$ 31	$ 31	$ 38	$ 38	$ 40	$ 37	$ 39	$ 129	[1]	$ 154	[1]	$ 126
Transmission revenues	16	17	16	16	17	18	18	17	65		70		69
Net income	30	35	33	39	38	41	36	42	137	[1]	157	[1]	137	[1]
Net income per common unit (in dollars per unit)	$ 0.56	$ 0.64	$ 0.60	$ 0.71	$ 0.70	$ 0.75	$ 0.69	$ 0.90	$ 2.51		$ 3.02		$ 2.91
Cash distribution paid	$ 43	$ 43	$ 42	$ 42	$ 42	$ 42	$ 35	$ 35
Quarterly financial other information
Ownership interest (as a percent)															25.00%	25.00%

[1] 25 percent interests in each of GTN and Bison were acquired in May 2011.