The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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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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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.

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Amount capitalized of allowance for funds used during construction.

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SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment
Depreciation expenses	$ 88.0	$ 78.0	$ 97.0
AFUDC - Equity	10.0	2.0	1.0
AFUDC debt	$ 1.3	$ 1.0	$ 1.0
Pipeline facilities and compression equipment \| Minimum
Property, Plant and Equipment
Estimated useful lives	20 years
Pipeline facilities and compression equipment \| Maximum
Property, Plant and Equipment
Estimated useful lives	68 years
Metering and other \| Minimum
Property, Plant and Equipment
Estimated useful lives	5 years
Metering and other \| Maximum
Property, Plant and Equipment
Estimated useful lives	77 years