PRESENTATION OF FINANCIAL INFORMATION - Jointly Owned Facilities (Details) $ in Thousands	9 Months Ended
Sep. 30, 2017 USD ($) facility
Jointly owned facilities
Number of jointly owned facilities \| facility	3
Electric plant in service	$ 845,908
Accumulated depreciation	614,820
Construction work in progress	$ 72,781
Yampa
Jointly owned facilities
Tri-State share (as a percent)	24.00%
Electric plant in service	$ 349,986
Accumulated depreciation	236,482
Construction work in progress	$ 48,627
MBPP
Jointly owned facilities
Tri-State share (as a percent)	24.13%
Electric plant in service	$ 413,234
Accumulated depreciation	298,440
Construction work in progress	$ 24,154
San Juan Project
Jointly owned facilities
Tri-State share (as a percent)	8.20%
Electric plant in service	$ 82,688
Accumulated depreciation	$ 79,898

Number of facilities jointly owned by the registrant.

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6500719&loc=d3e659983-123033

+ Details

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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.

+ References

No definition available.

+ Details

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

+ Details

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- Details

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- Details

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