SUBSEQUENT EVENTS (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended									1 Months Ended			12 Months Ended				1 Months Ended			1 Months Ended			1 Months Ended			1 Months Ended			1 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, 2009	Feb. 29, 2012 General Partner	May 31, 2011 General Partner	Nov. 30, 2009 General Partner	Dec. 31, 2011 General Partner	Jan. 31, 2012 General Partner	Jan. 31, 2012 TransCanada Corporation	Dec. 31, 2011 TransCanada Corporation	Feb. 29, 2012 Great Lakes	Feb. 02, 2012 Great Lakes	Dec. 31, 2011 Great Lakes	Feb. 29, 2012 Northern Border	Feb. 02, 2012 Northern Border	Dec. 31, 2011 Northern Border	Feb. 29, 2012 GTN	Feb. 02, 2012 GTN	Dec. 31, 2011 GTN	Feb. 29, 2012 Bison	Feb. 02, 2012 Bison	Dec. 31, 2011 Bison	Feb. 29, 2012 Cash distribution declared	Jan. 31, 2012 Cash distribution declared	Feb. 29, 2012 Cash Distribution Paid General Partner	Feb. 29, 2012 Cash Distribution Paid TransCanada Corporation	Feb. 29, 2012 Cash Distribution Paid Common unitholders	Feb. 29, 2012 Cash Distribution Paid Common units held by General Partner
Subsequent events
Cash distribution (in dollars per unit)																														$ 0.77
Cash distribution paid	$ 42.0	$ 42.0	$ 35.4	$ 35.4	$ 35.4	$ 34.4	$ 34.4	$ 34.4																					$ 42.0		$ 0.8	$ 8.7	$ 41.2	$ 4.5
Number of common units owned	53,500,000				46,200,000				46,200,000				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%
Aggregate distribution declared and payable by investee																	23.3			50.0			21.4			15.6
Ownership interest (as a percent)																		46.45%	46.45%		50.00%	50.00%		25.00%	25.00%		25.00%	25.00%
Partnership's share of distributions declared and payable by investee																	$ 10.8			$ 25.0			$ 5.4			$ 3.9