ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2017	Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2019
Regulatory assets and liabilities
Regulatory assets		$ 732,784		$ 497,279
Regulatory liabilities		122,051		122,169
Net regulatory asset		610,733		375,110
Impairment loss		259,761
Interest rate swaps, realized gain
Regulatory assets and liabilities
Regulatory liabilities		3,626		3,744
Proceeds from settlement of interest rate swaps	$ 4,600
Deferred non-member electric sales
Regulatory assets and liabilities
Regulatory liabilities		75,853		75,853
Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Regulatory liabilities		$ 42,572		42,572
First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029
Regulatory assets and liabilities
Term of issuance		12 years
Deferred income tax expense
Regulatory assets and liabilities
Regulatory assets		$ 33,970		58,937
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory assets and liabilities
Regulatory assets		83,142		83,714
Annual amortization expense		$ 2,300
Amortization period		47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets		$ 48,433		49,145
Annual amortization expense		$ 2,800
Amortization period		25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets		$ 36,936		37,194
Annual amortization expense		$ 1,000
Amortization period		44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets		$ 138,774		140,931
Annual amortization expense		$ 8,600
Amortization period		21 years 4 months 24 days
Deferred impairment loss | Unrecovered Plants
Regulatory assets and liabilities
Regulatory assets		$ 299,204		33,864
Deferred impairment loss | Nucla Generating Station
Regulatory assets and liabilities
Impairment loss			$ 37,100
Amortization period		3 years 3 months 18 days
Deferred impairment loss | Escalante Generating Station
Regulatory assets and liabilities
Impairment loss		$ 268,200
Deferred impairment loss | Holcomb Expansion
Regulatory assets and liabilities
Regulatory assets		92,325		$ 93,494
Annual amortization expense		$ 4,700
Amortization period		20 years