Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long-Term Debt

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- Definition

Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long-Term Debt, Hedged Interest Rate Risk Portion

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- Definition

Debt, Fair Value Adjustments During Period

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- Definition

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.

+ References

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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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-Name Accounting Standards Codification
-Topic 470
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-Section 50
-Paragraph 1
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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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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LONG-TERM DEBT - Schedule of Activity and Summary of Principal Repayments (Details)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2017 CAD	Jun. 30, 2017 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2016 CAD	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Outstanding at December 31			CAD 34,677,000,000	CAD 40,048,000,000
Current portion of long-term debt			(2,866,000,000)	(1,838,000,000)
Unamortized debt discount and issue costs			(174,000,000)	(191,000,000)
Fair value adjustments			238,000,000	293,000,000
Long-term debt, excluding current maturities			31,875,000,000	38,312,000,000
Decrease in fair value of interest rate hedge			4,000,000	0
Repayments of Long-term Debt [Abstract]
2018			2,866,000,000
2019			3,189,000,000
2020			2,834,000,000
2021			2,085,000,000
2022			1,929,000,000
Columbian Pipeline
Debt Instrument [Line Items]
Estimated increase (decrease) in fair value of acquired liabilities, long-term debt	CAD 300,000,000	$ 231,000,000	242,000,000	293,000,000
TRANSCANADA PIPELINES LIMITED
Debt Instrument [Line Items]
Outstanding at December 31			26,149,000,000	29,303,000,000
TRANSCANADA PIPELINES LIMITED \| Debentures, Maturity Dates Between 2018 and 2020
Debt Instrument [Line Items]
Outstanding at December 31			CAD 500,000,000	CAD 600,000,000
Interest Rate			10.80%	10.70%	10.80%		10.70%
TRANSCANADA PIPELINES LIMITED \| Debentures, Maturity Date of 2021
Debt Instrument [Line Items]
Outstanding at December 31			CAD 501,000,000	CAD 537,000,000	$ 400,000,000		$ 400,000,000
Interest Rate			9.90%	9.90%	9.90%		9.90%
TRANSCANADA PIPELINES LIMITED \| Medium Term Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 6,504,000,000	CAD 5,804,000,000
Interest Rate			4.90%	4.60%	4.90%		4.60%
TRANSCANADA PIPELINES LIMITED \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 18,644,000,000	CAD 19,660,000,000	$ 14,892,000,000		$ 14,642,000,000
Interest Rate			5.10%	5.10%	5.10%		5.10%
TRANSCANADA PIPELINES LIMITED \| Acquisition Bridge Facility
Debt Instrument [Line Items]
Outstanding at December 31			CAD 0	CAD 2,702,000,000	$ 0		$ 2,013,000,000
Interest Rate			0.00%	1.90%	0.00%		1.90%
NOVA GAS TRANSMISSION LTD.
Debt Instrument [Line Items]
Outstanding at December 31			CAD 895,000,000	CAD 917,000,000
NOVA GAS TRANSMISSION LTD. \| Debentures and Notes, Maturity Dates of 2024
Debt Instrument [Line Items]
Outstanding at December 31			CAD 100,000,000	CAD 100,000,000
Interest Rate			9.90%	9.90%	9.90%		9.90%
NOVA GAS TRANSMISSION LTD. \| Debentures and Notes, Maturity Date of 2023
Debt Instrument [Line Items]
Outstanding at December 31			CAD 250,000,000	CAD 269,000,000	$ 200,000,000		$ 200,000,000
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]
Outstanding at December 31			CAD 504,000,000	CAD 504,000,000
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]
Outstanding at December 31			CAD 41,000,000	CAD 44,000,000	$ 33,000,000		$ 33,000,000
Interest Rate			7.50%	7.50%	7.50%		7.50%
TRANSCANADA PIPELINE USA LTD. \| Acquisition Bridge Facility
Debt Instrument [Line Items]
Outstanding at December 31			CAD 0	CAD 2,283,000,000	$ 0		$ 1,700,000,000
Interest Rate			0.00%	1.90%	0.00%		1.90%
Columbian Pipeline \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 3,443,000,000	CAD 3,692,000,000	$ 2,750,000,000		$ 2,750,000,000
Interest Rate			4.00%	4.00%	4.00%		4.00%
TC PIPELINES, LP
Debt Instrument [Line Items]
Outstanding at December 31			CAD 2,573,000,000	CAD 2,054,000,000
TC PIPELINES, LP \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 1,502,000,000	CAD 940,000,000	$ 1,200,000,000		$ 700,000,000
Interest Rate			4.40%	4.70%	4.40%		4.70%
TC PIPELINES, LP \| Unsecured Loan Facility
Debt Instrument [Line Items]
Outstanding at December 31			CAD 232,000,000	CAD 215,000,000	$ 185,000,000		$ 160,000,000
Interest Rate			2.70%	1.90%	2.70%		1.90%
TC PIPELINES, LP \| Unsecured Term Loan
Debt Instrument [Line Items]
Outstanding at December 31			CAD 839,000,000	CAD 899,000,000	$ 670,000,000		$ 670,000,000
Interest Rate			2.70%	1.90%	2.70%		1.90%
TC PIPELINES, LP \| Unsecured Term Loan Maturing October 2020
Debt Instrument [Line Items]
Outstanding at December 31 \| $						$ 170,000,000
TC PIPELINES, LP \| Unsecured Term Loan Maturing October 2022
Debt Instrument [Line Items]
Outstanding at December 31 \| $						$ 500,000,000
ANR PIPELINE COMPANY \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 842,000,000	CAD 903,000,000	$ 672,000,000		$ 672,000,000
Interest Rate			7.20%	7.20%	7.20%		7.20%
GAS TRANSMISSION NORTHWEST LLC
Debt Instrument [Line Items]
Outstanding at December 31			CAD 382,000,000	CAD 423,000,000
GAS TRANSMISSION NORTHWEST LLC \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 313,000,000	CAD 336,000,000	$ 250,000,000		$ 250,000,000
Interest Rate			5.60%	5.60%	5.60%		5.60%
GAS TRANSMISSION NORTHWEST LLC \| Unsecured Term Loan
Debt Instrument [Line Items]
Outstanding at December 31			CAD 69,000,000	CAD 87,000,000	$ 55,000,000		$ 65,000,000
Interest Rate			1.10%	1.60%	1.10%		1.60%
GREAT LAKES GAS TRANSMISSION LIMITED PARTNERSHIP \| Senior Unsecured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 324,000,000	CAD 373,000,000	$ 259,000,000		$ 278,000,000
Interest Rate			7.70%	7.70%	7.70%		7.70%
PORTLAND NATURAL GAS TRANSMISSION SYSTEM \| Senior Secured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 38,000,000	CAD 71,000,000	$ 30,000,000		$ 53,000,000
Interest Rate			6.00%	6.00%	6.00%		6.00%
TUSCARORA GAS TRANSMISSION COMPANY
Debt Instrument [Line Items]
Outstanding at December 31			CAD 31,000,000	CAD 29,000,000
TUSCARORA GAS TRANSMISSION COMPANY \| Unsecured Term Loan
Debt Instrument [Line Items]
Outstanding at December 31			CAD 31,000,000	CAD 13,000,000	$ 25,000,000		$ 10,000,000
Interest Rate			1.10%	1.90%	1.10%		1.90%
TUSCARORA GAS TRANSMISSION COMPANY \| Senior Secured Notes
Debt Instrument [Line Items]
Outstanding at December 31			CAD 0	CAD 16,000,000	$ 0		$ 12,000,000
Interest Rate			0.00%	4.00%	0.00%		4.00%