LONG-TERM DEBT - Interest Rate Swaps (Details) $ in Thousands	9 Months Ended
Sep. 30, 2017 USD ($)
Interest rate swaps
Interest rate swaps
Notional Amount	$ 170,000
Interest Rate Swap - April 2016
Interest rate swaps
Notional Amount	$ 90,000
Fixed Rate	2.355%
Benchmark Interest Rate	30 year - LIBOR
Effective Date	Apr. 01, 2019
Maturity Date	Apr. 01, 2049
Interest Rate Swap - June 2016
Interest rate swaps
Notional Amount	$ 80,000
Fixed Rate	2.304%
Benchmark Interest Rate	30 year - LIBOR
Effective Date	Jun. 01, 2019
Maturity Date	Jun. 01, 2049

The date when a derivative contract becomes officially effective, in CCYY-MM-DD format.

No definition available.

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
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

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Date the derivative contract matures, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
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- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=tris_InterestRateSwapApril2016Member
Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
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