NET INCOME PER COMMON UNIT (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Common units and interest in the partnership
Net income	$ 36.1	[1]	$ 27.7	[1]	$ 78.4	[1]	$ 61.4	[1]
Net income allocated to General Partner:
General Partner interest	(0.7)		(0.5)		(1.6)		(1.2)
Incentive distribution income allocation
General Partner interest including Incentive distribution income	(0.7)		(0.5)		(1.6)		(1.2)
Net income allocable to common units	$ 35.4		$ 27.2		$ 76.8		$ 60.2
Weighted average common units outstanding (in units)	50.9		46.2		48.6		46.2
Net income per common unit (in dollars per unit)	$ 0.69		$ 0.59		$ 1.58		$ 1.30
General Partner
Common units and interest in the partnership
Additional percentage investment held by General Partner (as a percent)	2.00%				2.00%

[1] Includes equity earnings from GTN and Bison from May 3, 2011, date of acquisition, to June 30, 2011.