NET INCOME PER COMMON UNIT (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 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		Dec. 31, 2012 GTN	May 03, 2011 GTN	Dec. 31, 2012 Bison	May 03, 2011 Bison	Dec. 31, 2012 General Partner
Common units and interest in the partnership
Additional percentage investment held by General Partner																			2.00%
Ownership interest at the end of the period (as a percent)															25.00%	25.00%	25.00%	25.00%
Net income	$ 30	$ 35	$ 33	$ 39	$ 38	$ 41	$ 36	$ 42	$ 137	[1]	$ 157	[1]	$ 137	[1]
Net Income allocated to General Partner									(3)		(3)		(3)
Net Income allocable to common units									$ 134		$ 154		$ 134
Weighted average common units outstanding (millions) - basic (in units)									53.5		51.1		46.2
Weighted average common units outstanding (millions) - diluted (in units)									53.5		51.1		46.2
Net income per common unit - basic (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
Net income per common unit - diluted (in dollars per unit)									$ 2.51		$ 3.02		$ 2.91

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