PARTNERS' EQUITY (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended					12 Months Ended		1 Months Ended			12 Months Ended			12 Months Ended
	May 31, 2011	Nov. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Limited Partners	Dec. 31, 2010 Limited Partners	Feb. 29, 2012 General Partner	May 31, 2011 General Partner	Nov. 30, 2009 General Partner	Dec. 31, 2011 General Partner	Dec. 31, 2010 General Partner	Jan. 31, 2012 General Partner	Dec. 31, 2011 TransCanada Corporation	Jan. 31, 2012 TransCanada Corporation
Public offering
Number of common units sold in public offering	7,245,000	5,000,000
Price per common unit in public offering (in dollars per unit)	$ 47.58	$ 38.00
Gross proceeds from public offering of common units	$ 344.7	$ 190.0
Net proceeds from public offering of common units	330.9	181.8
Common units and interest in the partnership
Capital contribution by general partner									$ 6.7	$ 3.8
Aggregate ownership interest in the Partnership (as a percent)						98.00%					12.60%	14.30%		33.30%
Number of common units owned			53,500,000	46,200,000	46,200,000	53,472,766	46,227,766				5,797,106		5,797,106	11,287,725	11,287,725
General partner ownership interest in the partnership (as a percent)								2.00%	2.00%	2.00%	2.00%