INVESTMENTS IN UNCONSOLIDATED AFFILIATES (Details) (USD $)	3 Months Ended				9 Months Ended					3 Months Ended				9 Months Ended			1 Months Ended	3 Months Ended			9 Months Ended			3 Months Ended	4 Months Ended	9 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010	Dec. 31, 2011 Great Lakes	Sep. 30, 2011 Great Lakes	Mar. 31, 2011 Great Lakes	Sep. 30, 2010 Great Lakes	Sep. 30, 2011 Great Lakes	Sep. 30, 2010 Great Lakes	Dec. 31, 2010 Great Lakes	Apr. 30, 2006 Northern Border Y	Dec. 31, 2011 Northern Border	Sep. 30, 2011 Northern Border	Sep. 30, 2010 Northern Border	Sep. 30, 2011 Northern Border	Sep. 30, 2010 Northern Border	Dec. 31, 2010 Northern Border	Sep. 30, 2011 GTN	Sep. 30, 2011 GTN	Sep. 30, 2011 GTN	Sep. 30, 2011 Bison	Sep. 30, 2011 Bison	Sep. 30, 2011 Bison
Investments in unconsolidated affiliates
Ownership interest at the end of the period (as a percent)											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	$ 40,400,000	[1]	$ 35,600,000	[1]	$ 116,500,000	[1]	$ 91,800,000	[1]			$ 14,300,000		$ 14,600,000	$ 49,300,000	$ 44,000,000				$ 19,600,000	$ 21,000,000	$ 56,400,000	$ 47,800,000		$ 4,700,000		$ 7,100,000	$ 1,800,000		$ 3,700,000
Investment in Unconsolidated Affiliates	1,643,000,000				1,643,000,000				1,194,800,000		687,100,000			687,100,000		690,000,000			537,900,000		537,900,000		504,800,000	253,000,000	253,000,000	253,000,000	165,000,000	165,000,000	165,000,000
Amortization period of transaction fee (in years)																	12
Transaction fee																	10,000,000
Additional ownership interest acquired (as a percent)																	20.00%
Equity contribution										4,600,000		4,200,000		4,300,000	4,600,000			5,500,000	49,800,000		49,800,000
Total cash call issued by Great Lakes to fund its debt repayment										10,000,000		9,000,000
Revenues (expenses)
Transmission revenues											62,900,000		61,900,000	196,400,000	197,700,000				77,700,000	81,000,000	230,200,000	215,900,000		50,400,000	83,400,000		17,800,000	31,700,000
Operating expenses											(15,700,000)		(13,300,000)	(45,700,000)	(43,100,000)				(17,300,000)	(17,600,000)	(53,200,000)	(55,300,000)		(13,600,000)	(22,500,000)		(5,900,000)	(9,100,000)
Depreciation and amortization											(8,100,000)		(8,000,000)	(24,200,000)	(32,400,000)				(15,300,000)	(15,400,000)	(46,000,000)	(46,200,000)		(9,200,000)	(15,500,000)		(4,500,000)	(7,500,000)
Financial charges and other											(7,400,000)		(7,700,000)	(22,500,000)	(23,300,000)				(5,600,000)	(5,600,000)	(17,000,000)	(17,600,000)		(4,300,000)	(10,000,000)
Michigan business tax											(900,000)		(1,400,000)	2,100,000	(4,100,000)
Net income											30,800,000		31,500,000	106,100,000	94,800,000				39,500,000	42,400,000	114,000,000	96,800,000		23,300,000	35,400,000		7,400,000	15,100,000
Assets
Current assets											74,300,000			74,300,000		83,700,000			71,700,000		71,700,000		47,300,000	163,500,000	163,500,000	163,500,000	44,100,000	44,100,000	44,100,000
Plant, property and equipment, net											827,500,000			827,500,000		846,900,000			1,270,000,000		1,270,000,000		1,294,800,000	810,400,000	810,400,000	810,400,000	625,300,000	625,300,000	625,300,000
Other assets											600,000			600,000		600,000			28,300,000		28,300,000		22,900,000	3,500,000	3,500,000	3,500,000
Assets, total											902,400,000			902,400,000		931,200,000			1,370,000,000		1,370,000,000		1,365,000,000	977,400,000	977,400,000	977,400,000	669,400,000	669,400,000	669,400,000
Liabilities and Partners' Equity
Current liabilities											26,800,000			26,800,000		34,900,000			50,400,000		50,400,000		46,700,000	17,400,000	17,400,000	17,400,000	25,500,000	25,500,000	25,500,000
Deferred credits											400,000			400,000		5,600,000			11,500,000		11,500,000		9,700,000	19,600,000	19,600,000	19,600,000
Long-term debt, including current maturities											383,000,000			383,000,000		392,000,000			472,600,000		472,600,000		540,600,000	325,000,000	325,000,000	325,000,000
Partners' equity											492,200,000			492,200,000		498,700,000			835,500,000		835,500,000		768,000,000	615,400,000	615,400,000	615,400,000	643,900,000	643,900,000	643,900,000
Liabilities and Equity, total											902,400,000			902,400,000		931,200,000			1,370,000,000		1,370,000,000		1,365,000,000	977,400,000	977,400,000	977,400,000	669,400,000	669,400,000	669,400,000
Undistributed earnings																									$ 7,100,000			$ 3,700,000

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