SIGNIFICANT ACCOUNTING POLICIES - Income Taxes (Details) - PNGTS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Oregon Department of Revenue
Income Taxes
Current state and local tax expense (benefit)	$ 0.6
New Hampshire Department of Revenue Administration
Income Taxes
Effective income tax rate (as a percent)	3.00%	2.60%	3.50%
State and local income tax expense (benefit), continuing operations	$ 5.0	$ (1.0)	$ 1.0