LEASES - Leasing Arrangements as Lessee (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) lease MW	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Lease expenses
Operating lease expense	$ 900	$ 1,800
Number of financing leases | lease	0
Lessee operating lease assets and liabilities
Operating lease right-of-use assets, gross	$ 9,312
Less: Accumulated amortization	(985)
Net operating lease right-of-use assets	$ 8,327		$ 7,622
Operating lease right-of-use asset, Statement of Financial Position line item	us-gaap:DeferredCostsCurrentAndNoncurrent
Operating lease liabilities - current	$ (6,355)
Operating lease liabilities - current, Statement of Financial Position line item	us-gaap:LiabilitiesCurrent
Operating lease liabilities - noncurrent	$ (1,921)		$ (1,846)
Operating lease liabilities - noncurrent, Statement of Financial Position line item	tris:OtherDeferredCreditsAndOtherLiabilities
Total operating lease liabilities	$ (8,276)
Weighted average remaining lease term (years)	9 years 3 months 18 days
Weighted average discount rate	3.75%
Brush Generating Station
Operating Lease - Lessee
Megawatt usage | MW	70