LEASES - Leasing Arrangements as Lessee (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) lease MW	Sep. 30, 2018 USD ($)
Lease expenses
Operating lease expense	$ 1,800		$ 5,400
Rent expense		$ 1,900		$ 5,900
Number of financing leases | lease			0
Lessee operating lease assets and liabilities
Operating lease right-of-use assets, gross	7,941		$ 7,941
Less: Accumulated amortization	(4,143)		(4,143)
Net operating lease right-of-use assets	$ 3,798		$ 3,798
Operating lease right-of-use asset, Statement of Financial Position line item	us-gaap:DeferredCostsCurrentAndNoncurrent		us-gaap:DeferredCostsCurrentAndNoncurrent
Operating lease liabilities - current	$ 1,970		$ 1,970
Operating lease liabilities - current, Statement of Financial Position line item	us-gaap:LiabilitiesCurrent		us-gaap:LiabilitiesCurrent
Operating lease liabilities - noncurrent	$ 2,001		$ 2,001
Operating lease liabilities - noncurrent, Statement of Financial Position line item	tris:OtherDeferredCreditsAndOtherLiabilities		tris:OtherDeferredCreditsAndOtherLiabilities
Total operating lease liabilities	$ 3,971		$ 3,971
Weighted average remaining lease term (years)	4 years 9 months 18 days		4 years 9 months 18 days
Weighted average discount rate	5.58%		5.58%
Brush Generating Station
Operating Lease - Lessee
Megawatt usage | MW			70
Term of agreement	10 years		10 years