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VARIABLE INTEREST ENTITIES - Unconsolidated Entities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Unconsolidated Variable Interest Entities
Investment in other associations (in dollars)	$ 125,080	$ 123,686
Western Fuels Association \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments in other associations
Unconsolidated Variable Interest Entities
Investment in other associations (in dollars)	2,100	2,300
Trapper Mining \| Unconsolidated Variable Interest Entity - not primary beneficiary \| Investments in other associations
Unconsolidated Variable Interest Entities
Investment in coal mine	$ 14,400	$ 14,100
Western Fuels Association \| WFW \| Class AA shares
Unconsolidated Variable Interest Entities
Ownership interest held by parent (as a percent)	100.00%
Western Fuels Association \| WFW \| Class BB Shares
Unconsolidated Variable Interest Entities
Ownership interest held by parent (as a percent)	75.00%
MBPP
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	24.13%
MBPP \| Unconsolidated Variable Interest Entity - not primary beneficiary
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	24.13%
MBPP \| WFW \| Class BB Shares
Unconsolidated Variable Interest Entities
Ownership interest held by noncontrolling interest (as a percent)	25.00%
Yampa
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	24.00%
Yampa \| Trapper Mining \| Unconsolidated Variable Interest Entity - not primary beneficiary
Unconsolidated Variable Interest Entities
Tri-State share (as a percent)	26.57%