This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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The proportionate share of interest (percent) in a jointly owned utility plant.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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Net carrying amount of the assets and liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), when the reporting entity is not the VIE's primary beneficiary.

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VARIABLE INTEREST ENTITIES - Unconsolidated Entities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Western Fuels Association \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments in other associations
Unconsolidated Variable Interest Entities
Investment in unconsolidated VIE	$ 2.2	$ 2.4
WFW \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments in other associations
Unconsolidated Variable Interest Entities
Investment in unconsolidated VIE	$ 0.1	0.1
MBPP
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	27.13%
WFW \| Western Fuels Association \| Class AA shares
Unconsolidated Variable Interest Entities
Ownership interest held by parent (as a percent)	100.00%
WFW \| Western Fuels Association \| Class BB Shares
Unconsolidated Variable Interest Entities
Ownership interest held by parent (as a percent)	75.00%
WFW \| MBPP \| Class BB Shares
Unconsolidated Variable Interest Entities
Ownership interest held by noncontrolling interest (as a percent)	25.00%
MBPP \| Unconsolidated Variable Interest Entity - not primary beneficiary
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	27.13%
Trapper Mining \| Unconsolidated Variable Interest Entity - not primary beneficiary
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	26.57%
Trapper Mining \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments In and Advances To Coal Mines [Member]
Unconsolidated Variable Interest Entities
Investment in coal mine	$ 16.0	$ 15.9