INVESTMENTS IN UNCONSOLIDATED AFFILIATES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended					3 Months Ended				6 Months Ended			1 Months Ended		3 Months Ended			6 Months Ended			1 Months Ended	3 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010	Dec. 31, 2011 Great Lakes	Jun. 30, 2011 Great Lakes	Mar. 31, 2011 Great Lakes	Jun. 30, 2010 Great Lakes	Jun. 30, 2011 Great Lakes	Jun. 30, 2010 Great Lakes	Dec. 31, 2010 Great Lakes	Jul. 31, 2011 Northern Border	Apr. 30, 2006 Northern Border	Dec. 31, 2011 Northern Border	Jun. 30, 2011 Northern Border	Jun. 30, 2010 Northern Border	Jun. 30, 2011 Northern Border	Jun. 30, 2010 Northern Border	Dec. 31, 2010 Northern Border	Jun. 30, 2011 GTN	Jun. 30, 2011 GTN	Jun. 30, 2011 GTN	Jun. 30, 2011 Bison	Jun. 30, 2011 Bison	Jun. 30, 2011 Bison	Jun. 30, 2011 GTN and 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%	25.00%	25.00%
Equity Earnings from Unconsolidated Affiliates	$ 37.5	[1]	$ 25.3	[1]	$ 76.1	[1]	$ 56.2	[1]			$ 17.0		$ 13.1	$ 35.0	$ 29.4					$ 16.2	$ 12.2	$ 36.8	$ 26.8			$ 2.4	$ 2.4		$ 1.9	$ 1.9
Investment in Unconsolidated Affiliates	1,591.8				1,591.8				1,194.8		691.0			691.0		690.0				489.3		489.3		504.8	248.3	248.3	248.3	163.2	163.2	163.2
Amortization period of transaction fee (in years)																		12
Transaction fee																		10
Additional ownership interest acquired (as a percent)																		20.00%
Equity contribution										4.6		4.2		4.3	4.6		49.8		5.5												538.1
Total cash call issued by Great Lakes to fund its debt repayment										10.0	9.0
Revenues (expenses)
Transmission revenues											63.3		62.9	133.5	135.8					72.3	65.8	152.5	134.9
Operating expenses											(15.7)		(15.6)	(30.0)	(29.8)					(18.4)	(19.7)	(35.9)	(37.7)
Depreciation and amortization											(8.0)		(10.1)	(16.1)	(24.4)					(15.4)	(15.4)	(30.7)	(30.8)
Financial charges and other											(7.5)		(7.7)	(15.1)	(15.6)					(5.7)	(6.0)	(11.4)	(12.0)
Michigan business tax											4.6		(1.2)	3.0	(2.7)
Net income											36.7		28.3	75.3	63.3					32.8	24.7	74.5	54.4
Assets
Current assets											82.7			82.7		83.7				39.4		39.4		47.3
Plant, property and equipment, net											832.0			832.0		846.9				1,276.5		1,276.5		1,294.8
Other assets											0.6			0.6		0.6				22.6		22.6		22.9
Assets, total											915.3			915.3		931.2				1,338.5		1,338.5		1,365.0
Liabilities and Partners' Equity
Current liabilities											31.4			31.4		34.9				42.0		42.0		46.7
Deferred credits											0.4			0.4		5.6				10.9		10.9		9.7
Long-term debt, including current maturities											383.0			383.0		392.0				547.6		547.6		540.6
Partners' Equity											500.5			500.5		498.7				738.0		738.0		768.0
Liabilities and Equity, total											915.3			915.3		931.2				1,338.5		1,338.5		1,365.0
Undistributed earnings																									$ 2.4			$ 1.9

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