LONG-TERM DEBT - Schedule of Activity and Summary of Principal Repayments (Details) CAD in Millions, $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2015 USD ($)	Dec. 31, 2015 CAD
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 40,150		CAD 31,456
Less: Current portion of Long-term debt		1,838		2,547
Long-term debt, excluding current maturities		38,312		28,909
Repayments of Long-term Debt [Abstract]
2017		1,838
2018		8,941
2019		1,742
2020		2,762
2021		2,165
TRANSCANADA PIPELINES LIMITED
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		29,136		26,572
TRANSCANADA PIPELINES LIMITED \| Debentures, Maturity Dates Between 2017 and 2020
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 599		CAD 599
Interest Rate	10.70%	10.70%	10.70%	10.70%
TRANSCANADA PIPELINES LIMITED \| Debentures, Maturity Date of 2021
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 400	CAD 536	$ 400	CAD 553
Interest Rate	9.90%	9.90%	9.90%	9.90%
TRANSCANADA PIPELINES LIMITED \| Medium Term Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 5,787		CAD 5,175
Interest Rate	4.60%	4.60%	5.30%	5.30%
TRANSCANADA PIPELINES LIMITED \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 14,517	CAD 19,521	$ 14,641	CAD 20,245
Interest Rate	5.10%	5.10%	4.80%	4.80%
TRANSCANADA PIPELINES LIMITED \| Bridge Facility
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 2,006	CAD 2,693		CAD 0
Interest Rate	1.90%	1.90%	0.00%	0.00%
NOVA GAS TRANSMISSION LTD.
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 914		CAD 1,147
NOVA GAS TRANSMISSION LTD. \| Debentures and Notes, Maturity Dates between 2016 and 2024
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 100		CAD 324
Interest Rate	9.90%	9.90%	11.50%	11.50%
NOVA GAS TRANSMISSION LTD. \| Debentures and Notes, Maturity Date of 2023
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 200	CAD 268	$ 200	CAD 276
Interest Rate	7.90%	7.90%	7.90%	7.90%
NOVA GAS TRANSMISSION LTD. \| Medium-Term Notes, Maturity between 2025 and 2030
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 503		CAD 503
Interest Rate	7.40%	7.40%	7.40%	7.40%
NOVA GAS TRANSMISSION LTD. \| Medium-Term Notes, Maturity Date of 2026
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 33	CAD 43	$ 33	CAD 44
Interest Rate	7.50%	7.50%	7.50%	7.50%
TRANSCANADA PIPELINE USA LTD. \| Bridge Facility
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 1,695	CAD 2,276		CAD 0
Interest Rate	1.90%	1.90%	0.00%	0.00%
COLUMBIA PIPELINE GROUP, INC. \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 2,968	CAD 3,985		CAD 0
Interest Rate	3.70%	3.70%	0.00%	0.00%
ANR PIPELINE COMPANY \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 671	CAD 901	$ 432	CAD 597
Interest Rate	7.20%	7.20%	8.90%	8.90%
GAS TRANSMISSION NORTHWEST LLC
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 422		CAD 450
GAS TRANSMISSION NORTHWEST LLC \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 250	CAD 335	$ 250	CAD 346
Interest Rate	5.60%	5.60%	5.60%	5.60%
GAS TRANSMISSION NORTHWEST LLC \| Unsecured Term Loan
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 65	CAD 87	$ 75	CAD 104
Interest Rate	1.60%	1.60%	1.40%	1.40%
TC PIPELINES, LP
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 2,044		CAD 2,161
TC PIPELINES, LP \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 694	CAD 932	$ 694	CAD 957
Interest Rate	4.70%	4.70%	4.70%	4.70%
TC PIPELINES, LP \| Unsecured Loan Facility
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 158	CAD 213	$ 200	CAD 277
Interest Rate	1.90%	1.90%	1.60%	1.60%
TC PIPELINES, LP \| Unsecured Term Loan
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 670	CAD 899	$ 670	CAD 927
Interest Rate	1.90%	1.90%	1.60%	1.60%
GREAT LAKES GAS TRANSMISSION LIMITED PARTNERSHIP \| Senior Unsecured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 278	CAD 373	$ 297	CAD 411
Interest Rate	7.70%	7.70%	7.80%	7.80%
PORTLAND NATURAL GAS TRANSMISSION SYSTEM \| Senior Secured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 52	CAD 70	$ 69	CAD 96
Interest Rate	6.00%	6.00%	6.10%	6.10%
TUSCARORA GAS TRANSMISSION COMPANY
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31		CAD 29		CAD 22
TUSCARORA GAS TRANSMISSION COMPANY \| Unsecured Term Loan
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 10	CAD 13
Interest Rate	1.90%	1.90%	0.00%	0.00%
TUSCARORA GAS TRANSMISSION COMPANY \| Senior Secured Notes
Debt Instrument [Line Items]
Current and Long-Term Debt, Outstanding December 31	$ 12	CAD 16	$ 16	CAD 22
Interest Rate	4.00%	4.00%	4.00%	4.00%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 3
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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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