RELATED PARTY TRANSACTIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
General Partner
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	$ 2.2	$ 2.2	$ 2.1
TransCanada Corporation | Great Lakes
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	31.2	30.3	33.8
Impact on the Partnership's net income	14.1	12.8	14.3
Amounts payable to TransCanada and its affiliates	3.1	3.0
Transportation revenues from TransCanada and its affiliates	80.6	148.5	141.7
Percentage of transportation revenues to total revenues	32.20%	56.60%	48.90%
Affiliated rental revenue	1.3	0.9	0.6
Revenue from TransCanada and its affiliates included in the Partnership's equity income from Great Lakes	38.0	69.3	66.1
Great Lakes' receivables for transportation contracts with TransCanada and its affiliates	7.1	11.0
TransCanada Corporation | Great Lakes | Minimum
Costs charged to the pipeline system and amounts payable
TransCanada and its affiliates contracts' remaining term (in years)	1
TransCanada Corporation | Great Lakes | Maximum
Costs charged to the pipeline system and amounts payable
TransCanada and its affiliates contracts' remaining term (in years)	6
TransCanada Corporation | Northern Border
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	28.7	25.8	25.5
Impact on the Partnership's net income	13.4	12.5	12.3
Amounts payable to TransCanada and its affiliates	2.9	2.2
TransCanada Corporation | GTN
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	22.3
Impact on the Partnership's net income	21.2
Amounts payable to TransCanada and its affiliates	3.0
TransCanada Corporation | Bison
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	7.7
Impact on the Partnership's net income	4.3
Amounts payable to TransCanada and its affiliates	1.0
TransCanada Corporation | North Baja Pipeline, LLC
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	3.7	4.4	2.9
Impact on the Partnership's net income	3.5	3.2	2.4
Amounts payable to TransCanada and its affiliates	0.5	0.6
TransCanada Corporation | Tuscarora Gas Transmission Company
Costs charged to the pipeline system and amounts payable
Costs charged by TransCanada and its affiliates	4.7	3.7	3.0
Impact on the Partnership's net income	4.6	3.5	2.8
Amounts payable to TransCanada and its affiliates	$ 0.6	$ 0.7