Name:	tris_LongTermContractForPurchaseOfElectricPowerElectiveShareOfRequirementsFromDistributedOrRenewableGenerationProvidedByMemberMaximum
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Data Type:	num:percentItemType
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The minimum period of time required for notice of termination under terms of long term contract.

No definition available.

Name:	tris_LongTermContractTerminationNoticePeriodMinimum
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Data Type:	xbrli:durationItemType
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Number of members of the reporting entity.

No definition available.

Name:	tris_NumberOfMembers
Namespace Prefix:	tris_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Number of members exercising election to provide up to five percent of transmission or renewable generation from entities owned or controlled by the member.

No definition available.

Name:	tris_NumberOfMembersExercisingElection
Namespace Prefix:	tris_
Data Type:	xbrli:integerItemType
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Period Type:	duration

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

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Data Type:	num:percentItemType
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Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.