VARIABLE INTEREST ENTITIES - Unconsolidated Entities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Unconsolidated Variable Interest Entities
Investment in other associations (in dollars)	$ 143,608	$ 139,350
Western Fuels Association
Unconsolidated Variable Interest Entities
Investment in other associations (in dollars)	2,346	2,245
Western Fuels Association \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments in other associations
Unconsolidated Variable Interest Entities
Investment in other associations (in dollars)	$ 2,300	2,200
MBPP
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	24.13%
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)	24.13%
Trapper Mining \| Unconsolidated Variable Interest Entity - not primary beneficiary
Unconsolidated Variable Interest Entities
Investment in coal mine	$ 15,000	$ 14,500
Tri-State share (as a percent)	26.57%

Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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