QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Quarterly financial data (unaudited)
Equity income	$ 37.0	$ 40.4	$ 37.5	$ 38.6	$ 34.2	$ 35.6	$ 25.3	$ 30.9	$ 153.5	[1]	$ 126.0	[1]	$ 99.4	[1]
Transmission revenues	17.9	17.6	17.6	17.3	17.3	17.4	17.0	17.4	70.4		69.1		67.9
Net income	38.3	40.7	36.1	42.3	37.1	38.6	27.7	33.7	157.4	[1],[2]	137.1	[2]	106.1	[2],[3]
Net income per common unit (in dollars per unit)	$ 0.70	$ 0.75	$ 0.69	$ 0.90	$ 0.79	$ 0.82	$ 0.59	$ 0.71	$ 3.02		$ 2.91		$ 2.34
Cash distribution paid	$ 42.0	$ 42.0	$ 35.4	$ 35.4	$ 35.4	$ 34.4	$ 34.4	$ 34.4

[1] Includes equity earnings from GTN and Bison from May 3, 2011, date of acquisition, to December 31, 2011.

[2] Recast as discussed in Notes 2 and 5 and includes equity earnings from GTN and Bison from May 3, 2011, date of acquisition, to December 31, 2011.

[3] Recast as discussed in Notes 2 and 5.