Amount of cash outflow in the form amortization of derivatives loss.

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Rate to unwind an interest rate swap that is used to hedged a credit facility.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Average fixed interest rate related to the group of interest rate derivatives.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

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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, after tax and before reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount, after tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount, after tax, of adjustments from accumulated other comprehensive income (AOCI) for gain (loss) from increase (decrease) in value of excluded component of derivative designated and qualifying as hedge. Adjustments include, but are not limited to, reclassifications for sale and settlement, and amounts recognized under systematic and rational method.

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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The cash outflow to repay long-term debt that is wholly or partially secured by collateral. Excludes repayments of tax exempt secured debt.

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FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jun. 26, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Interest rate derivatives
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss)			$ (2)		$ (2)
Reclassification of net income of gains and losses on cash flow hedges		$ 1	3	$ 1	3
Net realized gain related to the interest rate swaps included in financial charges and other					2
Amortization of derivatives loss			2		2
Change in fair value of interest rate derivative instruments recognized in other comprehensive income		$ (9)	(1)	$ (14)	6
2013 Term Loan Facility
Interest rate derivatives
Repayments of secured debt	$ 50
Unwind rate of interest rate swap (as a percent)	2.81%
Interest rate swaps \| Term loan \| TC PipeLines, LP 2013 Term Loan Facility due 2022
Interest rate derivatives
Weighted average fixed interest rate (as a percent)		3.26%		3.26%
Hedges of cash flows \| Interest rate swaps
Interest rate derivatives
Change in fair value of interest rate derivative instruments recognized in other comprehensive income		$ (9)	(1)	$ (14)	6
Hedges of cash flows \| Interest rate swaps \| Financial charges and other
Interest rate derivatives
Net realized gain related to the interest rate swaps included in financial charges and other		0	$ 1	1	$ 2
Hedges of cash flows \| Recurring fair value measurement \| Level 2 \| Interest rate swaps
Interest rate derivatives
Fair value of derivative asset, gross						$ 8
Fair value of derivative liability, gross		5		5
Fair value of derivative asset, net						$ 8
Fair value of derivative liability, net		$ 5		$ 5