PROPERTY, PLANT AND EQUIPMENT - Jointly Owned Facilities (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Jointly owned facilities
Electric plant in service	$ 817,974
Accumulated depreciation	579,219
Construction work in progress	$ 37,871
Yampa
Jointly owned facilities
Tri-State share (as a percent)	24.00%
Electric plant in service	$ 345,937
Accumulated depreciation	229,464
Construction work in progress	$ 23,518
MBPP
Jointly owned facilities
Tri-State share (as a percent)	24.13%
Electric plant in service	$ 389,283
Accumulated depreciation	288,386
Construction work in progress	$ 13,791
San Juan Project
Jointly owned facilities
Tri-State share (as a percent)	8.20%
Electric plant in service	$ 82,754
Accumulated depreciation	61,369
Construction work in progress	$ 562

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

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 10
-Section C

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

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

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

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