NET INCOME PER COMMON UNIT (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended				1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		May 31, 2011 General Partner	Sep. 30, 2011 General Partner	Sep. 30, 2011 General Partner
Common units and interest in the partnership
Additional percentage investment held by General Partner (as a percent)									2.00%	2.00%	2.00%
Net income	$ 40.7	[1]	$ 38.6	[1]	$ 119.1	[1]	$ 100.0	[1]
Net income allocated to General Partner:
General Partner interest	(0.8)		(0.8)		(2.4)		(2.0)
General Partner interest including Incentive distribution income	(0.8)		(0.8)		(2.4)		(2.0)
Net income allocable to common units	$ 39.9		$ 37.8		$ 116.7		$ 98.0
Weighted average common units outstanding	53.5		46.2		50.2		46.2
Net income per common unit (in dollars per unit)	$ 0.75		$ 0.82		$ 2.33		$ 2.12

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