SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Regulatory Assets and Liabilities (Details) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) facility	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Regulatory assets and liabilities
Number of jointly owned facilities | facility					2
Regulatory assets and liabilities
Regulatory assets					$ 497,279	$ 437,377
Regulatory liabilities					122,169	137,369
Net regulatory asset					$ 375,110	300,008
Impairment loss						0		$ 0
Proceeds from settlement of interest rate swaps							$ 4,625
First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029
Regulatory assets and liabilities
Term of issuance					12 years
Nucla Generating Station
Regulatory assets and liabilities
Impairment loss					$ 37,100
Holcomb Expansion
Regulatory assets and liabilities
Impairment loss							$ 93,500
Interest rate swap - unrealized gain
Regulatory assets and liabilities
Regulatory liabilities						8,576
Gain recognized on termination of interest rate swap agreement	$ 0
Interest rate swaps, realized gain
Regulatory assets and liabilities
Regulatory liabilities					3,744	4,215
Proceeds from settlement of interest rate swaps		$ 4,600
Deferred non-member electric sales
Regulatory assets and liabilities
Regulatory liabilities					75,853	82,006
Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Regulatory liabilities					42,572	42,572
Deferred income tax expense
Regulatory assets and liabilities
Regulatory assets					58,937	18,098
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory assets and liabilities
Regulatory assets					83,714	86,005
Annual amortization expense					$ 2,300
Amortization period					47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets					$ 49,145	51,994
Annual amortization expense					$ 2,800
Amortization period					25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets					$ 37,194	38,227
Annual amortization expense					$ 1,000
Amortization period					44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets					$ 140,931	149,559
Annual amortization expense					$ 8,600
Amortization period					21 years 4 months 24 days
Deferred impairment loss | Nucla Generating Station
Regulatory assets and liabilities
Regulatory assets					$ 33,864
Amortization period					3 years 3 months 18 days
Impairment loss			$ 37,100
Deferred impairment loss | Holcomb Expansion
Regulatory assets and liabilities
Regulatory assets					$ 93,494	$ 93,494
Annual amortization expense				$ 4,700
Amortization period				20 years