SUBSEQUENT EVENTS (Details) (USD $) In Millions, except Per Share data, unless otherwise specified		1 Months Ended			1 Months Ended		1 Months Ended				1 Months Ended
	Jun. 30, 2011 Great Lakes	Aug. 31, 2011 Great Lakes Distribution subsequently declared	Aug. 01, 2011 Great Lakes Distribution subsequently declared	Jun. 30, 2011 Northern Border	Aug. 31, 2011 Northern Border Distribution subsequently declared	Aug. 01, 2011 Northern Border Distribution subsequently declared	Jul. 31, 2011 Northern Border Equity Method Investments	Jul. 27, 2011 Northern Border Equity Method Investments	Jun. 30, 2011 GTN	Jun. 30, 2011 Bison	Jul. 31, 2011 Senior revolving credit facility Issuance of debt	Jul. 31, 2011 Distribution subsequently declared	Jul. 31, 2011 Senior revolving credit facility	Jun. 30, 2011 Senior revolving credit facility	Jun. 30, 2011 Senior term loan
Subsequent events
Line of credit facility, additonal amount											$ 500.0		$ 500.0
Line of credit facility, base rate											LIBOR-based interest rate plus a margin		London Interbank Offered Rate (LIBOR)-based interest rate plus a margin
Maximum borrowing capacity														250.0	300.0
Cash distribution (in dollars per unit)												$ 0.77
Equity contribution							49.8
Ownership interest (as a percent)	46.45%		46.45%	50.00%		50.00%		50.00%	25.00%	25.00%
Aggregate distribution declared and payable by investee		39.1			41.6
Cash call issued to fund capitalization requirements							99.6
Partnership's share of distributions declared and payable by investee		$ 18.2			$ 20.8