FAIR VALUE MEASUREMENTS - Interest Rate Swaps (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016		Jun. 30, 2017	Jun. 30, 2016		Dec. 31, 2016
Interest rate derivatives
Fair value of derivative asset, net							$ 0.0
Amortization of realized loss on derivative financial instruments	$ 1.0	$ 1.0	[1]	$ 1.0	$ 1.0	[1]
Amortization of derivatives loss	$ 1.0	1.0		1.0	1.0
Portland Natural Gas Transmission System
Interest rate derivatives
Payments for derivative instruments				$ 20.9
Interest acquired by Partnership (as a percent)	61.71%			61.71%
Net unamortized loss included in other comprehensive income	$ 1.0			$ 1.0			2.0
Amortization of derivatives loss				$ 1.0	1.0
Interest rate swaps | Term loan | TC PipeLines, LP 2013 Term Loan Facility due July 2018
Interest rate derivatives
Weighted average fixed interest rate (as a percent)	2.31%			2.31%
Hedges of cash flows | Interest rate swaps
Interest rate derivatives
Amortization of realized loss on derivative financial instruments	$ 0.0	(1.0)		$ 1.0	(3.0)
Hedges of cash flows | Interest rate swaps | Financial charges and other
Interest rate derivatives
Net realized loss related to the interest rate swaps	0.0	$ 1.0		0.0	$ 1.0
Hedges of cash flows | Interest rate swaps | Recurring fair value measurement | Level 2
Interest rate derivatives
Fair value of derivative asset, net	2.0			2.0
Designated as hedge | Interest rate swaps | Recurring fair value measurement | Level 2
Interest rate derivatives
Fair value of derivative asset, gross	$ 2.0			$ 2.0
Fair value of derivative liability, gross							0.0
Designated as hedge | Hedges of cash flows | Interest rate swaps | Level 2
Interest rate derivatives
Fair value of derivative asset, gross							1.0
Fair value of derivative liability, gross							$ 1.0

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).