RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - OTHER FINANCIAL INSTRUMENTS (Details) - CAD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair Value of Other Financial Instruments
Equity investments at carrying value	$ 102,000,000	$ 99,000,000
Restricted long-term investments (Note 14)	323,000,000	267,000,000
Long-term debt	64,400,000,000	67,400,000,000
Notes receivable, noncurrent	97,000,000	89,000,000
Notes receivable, fair value	97,000,000	89,000,000
Net Investment Hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt	479,000,000	367,000,000
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts	30,000,000	286,000,000
Realized loss associated with the settlement of foreign exchange forward contracts	45,000,000	198,000,000
Realized gain (loss) associated with the settlement of United Stated dollar denominated debt that matured	14,000,000	23,000,000
Amount of ineffectiveness	0	0
Carrying value
Fair Value of Other Financial Instruments
Long-term debt	63,900,000,000	64,000,000,000
Preference shares
Fair Value of Other Financial Instruments
Held-to-maturity securities	$ 478,000,000	371,000,000
Cumulative dividends based on average yield of Government of Canada bonds, maturity period of bonds	10 years
Held to maturity preferred share investment	$ 580,000,000	$ 580,000,000
Noverco \| Preference shares
Fair Value of Other Financial Instruments
Cumulative dividends based on average yield of Government of Canada bonds, spread over reference rate (as a percent)	4.38%

Represents the percentage points added to the variable reference rate for the calculation of cumulative dividends.

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Represents the maturity period of the Government of Canada bonds that are used as the variable reference rate for the calculation of cumulative dividends.

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Gain (loss) on hedging derivative instrument or the foreign currency transaction gain (loss) on the nonderivative hedging instrument designated as a hedge of a net investment in foreign operations related to hedge ineffectiveness, which is included in earnings in the period.

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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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Aggregate impact of foreign currency derivatives on earnings for the period. Includes impact of foreign currency cash flow hedge ineffectiveness and the ineffectiveness of fair value hedging derivatives and full change in fair value of the foreign currency derivatives not designated as hedging instruments.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

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Amounts due from parties associated with the reporting entity as evidenced by a written promise to pay, due after 1 year (or 1 business cycle).

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Amount before tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income for translation gain (loss) realized upon the sale or liquidation of an investment in a foreign entity and foreign currency hedges that are designated and qualified as hedging instruments for hedges of the foreign currency exposure of a net investment in a foreign operation.

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The aggregate value of all restricted investments.

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Amount of net unrealized gain (loss) related to the change in fair value of foreign currency exchange rate derivatives designated as cash flow hedging instruments. Recorded in accumulated other comprehensive income to the extent that the cash flow hedge is determined to be effective.

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