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

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The cash outflow for settlement of derivative instruments during the period, which are designated as hedging instruments.

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Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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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.

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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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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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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 before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	May 10, 2018	Dec. 31, 2017
Interest rate derivatives
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss)			$ 2.0	$ 1.0
Debt and credit facilities	$ 2,258.0		2,258.0				$ 2,415.0
Amortization of derivatives loss			2.0	1.0
Term loan \| TC PipeLines, LP 2013 Term Loan Facility due 2022
Interest rate derivatives
Debt and credit facilities	500.0		500.0				500.0
Portland Natural Gas Transmission System
Interest rate derivatives
Payments for derivative instruments			20.9
Interest acquired (as a percent)						61.71%
Net unamortized loss included in other comprehensive income							1.0
Amortization of derivatives loss	$ 0.0	$ 0.0	$ 1.0	1.0
Interest rate swaps \| Term loan \| 2013 Term Loan Facility due 2018
Interest rate derivatives
Weighted average fixed interest rate (as a percent)	3.26%		3.26%		2.31%
Hedges of cash flows \| Interest rate swaps
Interest rate derivatives
Change in fair value of interest rate derivative instruments recognized in other comprehensive income (loss)	$ 3.0	0.0	$ 12.0	1.0
Net realized gain (loss) on reclassified from other comprehensive income	(1.0)	(1.0)	(4.0)	0.0
Hedges of cash flows \| Interest rate swaps \| Financial charges and other
Interest rate derivatives
Net realized gain (loss) on reclassified from other comprehensive income	0.0	$ 0.0	(2.0)	$ 0.0
Hedges of cash flows \| Interest rate swaps \| Recurring fair value measurement \| Level 2
Interest rate derivatives
Fair value of derivative asset, gross							5.0
Fair value of derivative asset, net							$ 5.0
Designated as hedge \| Interest rate swaps \| Recurring fair value measurement \| Level 2
Interest rate derivatives
Fair value of derivative asset, gross	17.0		17.0
Fair value of derivative asset, net	$ 17.0		$ 17.0