PROPERTY, PLANT AND EQUIPMENT - Jointly Owned Facilities (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Jointly owned facilities
Electric plant in service	$ 886,448
Accumulated depreciation	544,188
Construction work in progress	$ 1,512
Yampa
Jointly owned facilities
Tri-State share (as a percent)	24.00%
Electric plant in service	$ 396,292
Accumulated depreciation	245,723
Construction work in progress	$ 21
MBPP
Jointly owned facilities
Tri-State share (as a percent)	27.13%
Electric plant in service	$ 490,156
Accumulated depreciation	298,465
Construction work in progress	$ 1,491

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

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- Definition

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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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

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- Definition

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

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