ACQUISITIONS AND REVISED INCENTIVE DISTRIBUTION RIGHTS (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		12 Months Ended			1 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended					1 Months Ended		12 Months Ended			1 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended
	May 31, 2011	Nov. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	May 31, 2011 Bridge loan facility	May 03, 2011 Bridge loan facility	Jul. 13, 2011 Senior revolving credit facility	May 31, 2011 GTN and Bison	Dec. 31, 2011 GTN and Bison	May 03, 2011 GTN and Bison Bridge loan facility	May 03, 2011 GTN and Bison Senior revolving credit facility	May 31, 2011 GTN and Bison TransCanada Subsidiaries	Dec. 31, 2011 GTN and Bison TransCanada Subsidiaries	May 03, 2011 GTN and Bison TransCanada Subsidiaries	May 03, 2011 GTN	May 03, 2011 GTN TransCanada Subsidiaries	May 03, 2011 Bison	May 31, 2011 Bison TransCanada Subsidiaries	May 03, 2011 Bison TransCanada Subsidiaries	Dec. 31, 2011 Yuma Lateral Asset Acquisition TransCanada Corporation	Mar. 25, 2011 Yuma Lateral Asset Acquisition TransCanada Corporation	Mar. 05, 2010 Yuma Lateral Asset Acquisition TransCanada Corporation	Jul. 31, 2009 Yuma Lateral Asset Acquisition TransCanada Corporation Maximum	Mar. 05, 2010 Yuma Lateral Asset Acquisition TransCanada Corporation Maximum	Jul. 02, 2009 North Baja Acquisition Senior revolving credit facility	Jul. 31, 2009 North Baja Acquisition TransCanada Corporation	Dec. 31, 2009 North Baja Acquisition TransCanada Corporation	Jul. 02, 2009 North Baja Acquisition TransCanada Corporation	Jul. 31, 2009 North Baja Acquisition General Partner
Asset acquisition
Interest acquired by the Partnership (as a percent)																	25.00%			25.00%									100.00%
Purchase price															$ 605.0		$ 405.0			$ 200.0			$ 7.6						$ 271.4
Acquired debt																	81.3
Percentage of acquired debt																	25.00%
Future commitments related to acquisitions																			9.1					10.0
Number of common units sold in public offering	7,245,000	5,000,000							7,245,000
Price per common unit issued (in dollars per unit)	$ 47.58	$ 38.00							$ 47.58
Net proceeds from issuance of common units	330.9	181.8							330.9
Amount drawn on credit facility to finance acquisition											61.0	125.0														170.0
Outstanding borrowing capacity						61.0
Maximum borrowing capacity							400.0	500.0			400.0	250.0
Common units issued to TransCanada			7,300,000		11,300,000																						2,609,680
Price per common unit issued to TransCanada																													$ 30.042
Net proceeds from issuance of common units to TransCanada																													78.4
Contribution by General Partners													6.7														1.6
General partner ownership interest in the partnership (as a percent)																														2.00%
Cash on hand															15.1														21.4
Additional payment to TransCanada										23.5
Equity investment																246.2		161.3			10.0
Cash paid														538.7								2.4
Difference between purchase price and net assets acquired			133.6	(0.1)	27.7									131.2							2.3						27.7
Payment to be made to TransCanada																									2.4
Purchase price recorded as working capital										0.6																			2.0
Purchase price recorded as plant, property and equipment																					7.7								193.5
Purchase price recorded as goodwill																													48.5
Purchase price recorded as other assets																													0.1
Purchase price recorded as Other long-term liabilities																													0.4
Increase in Partnership's net income																												$ 8.3