LEASES - Leasing Arrangements as Lessee (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) lease MW	Dec. 31, 2018 USD ($)
Lease expenses
Operating lease expense	$ 7,400
Rent expense		$ 7,900
Number of financing leases | lease	0
Lessee operating lease assets and liabilities
Operating lease right-of-use assets, gross	$ 8,376
Less: Accumulated amortization	(754)
Net operating lease right-of-use assets	$ 7,622
Operating lease right-of-use asset, Statement of Financial Position line item	us-gaap:DeferredCostsCurrentAndNoncurrent
Operating lease liabilities - current	$ (5,533)
Operating lease liabilities - current, Statement of Financial Position line item	us-gaap:LiabilitiesCurrent
Operating lease liabilities - noncurrent	$ (1,846)
Operating lease liabilities - noncurrent, Statement of Financial Position line item	tris:OtherDeferredCreditsAndOtherLiabilities
Total operating lease liabilities	$ (7,379)
Weighted average remaining lease term (years)	9 years 6 months
Weighted average discount rate	3.80%
Brush Generating Station
Operating Lease - Lessee
Megawatt usage | MW	70