INVESTMENTS IN UNCONSOLIDATED AFFILIATES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended								3 Months Ended		12 Months Ended				12 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended						12 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, 2011		Dec. 31, 2010		Dec. 31, 2009		Dec. 31, 2011 TC GL Intermediate Limited Partnership	Dec. 31, 2011 TC PipeLines Intermediate Limited Partnership	Dec. 31, 2011 Great Lakes	Mar. 31, 2011 Great Lakes	Dec. 31, 2011 Great Lakes	Dec. 31, 2010 Great Lakes	Dec. 31, 2009 Great Lakes	Feb. 28, 2007 Great Lakes	Dec. 31, 2011 Great Lakes TransCanada Corporation	Apr. 30, 2006 Northern Border	Dec. 31, 2011 Northern Border	Sep. 30, 2011 Northern Border	Dec. 31, 2011 Northern Border Y	Dec. 31, 2010 Northern Border	Dec. 31, 2009 Northern Border	Dec. 31, 2011 Northern Border ONEOK Partners L.P.	Dec. 31, 2011 GTN	May 03, 2011 GTN	Dec. 31, 2011 Bison	May 03, 2011 Bison
Investments in unconsolidated affiliates
Ownership interest at the end of the period (as a percent)																	46.45%		46.45%			46.45%			50.00%		50.00%				25.00%	25.00%	25.00%	25.00%
Partnership interest held (as a percent)															98.9899%	98.9899%
Equity income	$ 37.0	$ 40.4	$ 37.5	$ 38.6	$ 34.2	$ 35.6	$ 25.3	$ 30.9	$ 153.5	[1]	$ 126.0	[1]	$ 99.4	[1]					$ 59.5	$ 58.7	$ 59.1						$ 75.5	$ 67.3	$ 40.3		$ 11.5		$ 7.0
Undistributed earnings																																	1.5
Investment in Unconsolidated Affiliates	1,609.5				1,194.8				1,609.5		1,194.8						685.5		685.5	690.0					536.1		536.1	504.8			225.1		162.8
Amortization period of transaction fee (in years)																											12
Transaction fee																								10
Additional ownership interest acquired (as a percent)																								20.00%
Limited partnership interest (as a percent)																							53.55%							50.00%
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill																	458.4		458.4	458.4					119.9		119.9	120.8
Equity contribution																	4.6	4.2	8.9	9.3	0.1				5.0	49.8	54.8		42.3
Total cash call issued by Great Lakes to fund its debt repayment																	10.0	9.0
Assets
Current assets																	65.3		65.3	83.7											54.6		9.9
Cash and cash equivalents																									32.8		32.8	10.2
Other current assets																									35.6		35.6	37.1
Plant, property and equipment, net																	826.2		826.2	846.9					1,266.6		1,266.6	1,294.8			1,207.2		657.9
Other assets																	0.6		0.6	0.6					31.4		31.4	22.9			1.1
Assets, total																	892.1		892.1	931.2					1,366.4		1,366.4	1,365.0			1,262.9		667.8
Liabilities and Partners' Equity
Current liabilities																	30.0		30.0	34.9					48.6		48.6	46.7			17.7		16.8
Deferred credits and other																	0.4		0.4	5.6					12.8		12.8	9.7			19.7
Long-term debt, including current maturities																	373.0		373.0	392.0					472.6		472.6	540.6			325.0
Partners' equity
Partners' capital																	488.7		488.7	498.7					835.1		835.1	770.9			900.5		651.0
Accumulated other comprehensive loss																									(2.7)		(2.7)	(2.9)
Liabilities and Equity, total																	892.1		892.1	931.2					1,366.4		1,366.4	1,365.0			1,262.9		667.8
Revenues (expenses)
Transmission revenues																			250.0	262.4	289.7						310.1	295.1	249.2		133.2		51.8
Operating expenses																			(61.8)	(59.2)	(66.5)						(73.2)	(74.0)	(70.8)		(36.7)		(11.3)
Depreciation																			(32.2)	(40.5)	(58.5)						(61.6)	(61.5)	(61.9)		(36.2)		(12.4)
Financial charges and other																			(29.9)	(30.9)	(31.9)						(22.6)	(23.4)	(34.4)		(15.0)
Michigan business tax																			1.9	(5.3)	(5.4)
Net income																			$ 128.0	$ 126.5	$ 127.4						$ 152.7	$ 136.2	$ 82.1		$ 45.3		$ 28.1

[1] Includes equity earnings from GTN and Bison from May 3, 2011, date of acquisition, to December 31, 2011.