Face (par) amount of the original debt instrument that was exchanged.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The period of time the terms of a letter of credit are effective.

+ References

No definition available.

+ Details

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Namespace Prefix:	tris_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from exchange of debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

No definition available.

+ Details

Name:	tris_ProceedsFromExchangeOfLongTermDebt
Namespace Prefix:	tris_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=tris_FirstMortgageBondsSeries2014eMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegistrationPaymentArrangementByArrangementAxis=tris_RegistrationRightsAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=tris_PollutionControlRevenueBondsMoffatCountyColoradoVariableRateDemandSeries2009Due2036Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=tris_RevolvingCreditAgreementDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=tris_RevolvingCreditAgreementDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=tris_BankOfAmericaMember
Namespace Prefix:
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LONG-TERM DEBT - Other Information (Details) - USD ($)	1 Months Ended
LONG-TERM DEBT - Other Information (Details) - USD ($)	Nov. 30, 2015	Sep. 30, 2015	Dec. 31, 2015	Jul. 30, 2015	Dec. 31, 2014	Oct. 30, 2014
Debt
Outstanding borrowings			$ 3,371,679,000		$ 3,242,470,000
Series 2014 Bonds
Debt
Face amount of debt issued						$ 500,000,000
Series 2014 Bonds \| Registration Rights Agreement
Debt
Face amount of debt exchanged				$ 500,000,000
Proceeds from Exchange of Long-term Debt		$ 0
Pollution control revenue bonds, Moffat County, CO Variable Rate Demand Series 2009, due 2036
Debt
Outstanding borrowings			46,800,000		46,800,000
Revolving Credit Agreement
Debt
Maximum borrowing capacity			750,000,000
Outstanding borrowings			271,000,000		$ 50,000,000
Available borrowing capacity			$ 431,000,000
Revolving Credit Agreement \| Pollution control revenue bonds, Moffat County, CO Variable Rate Demand Series 2009, due 2036 \| Bank of America
Debt
Letter of credit, period	364 days