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The number of plans sponsored by entity.

No definition available.

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The required age to qualify for participation and/or receive particular benefits of the plan.

No definition available.

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The threshold percentage, represented by unrecognized amount of actuarial gains and losses as a percentage of accumulated postretirement benefit obligation, above which the amounts are reclassified out of AOCI.

No definition available.

Name:	tris_DefinedBenefitPlanThresholdUnrecognizedAmountOfActuarialGainsAndLossesAsPercentageOfAccumulatedPostretirementBenefitObligation
Namespace Prefix:	tris_
Data Type:	num:percentItemType
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Period Type:	instant

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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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.

Name:	us-gaap_DefinedContributionPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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Name:	us-gaap_RetirementPlanTypeAxis=tris_MedicalPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RetirementPlanTypeAxis=tris_PostretirementMedicalBenefitPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RetirementPlanTypeAxis=tris_PostemploymentMedicalBenefitPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CollectiveBargainingArrangementAxis=tris_NonBargainingUnitEmployeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: