ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Jun. 30, 2019	Oct. 31, 2017	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Regulatory assets and liabilities
Regulatory assets			$ 462,038	$ 462,038	$ 437,377
Regulatory liabilities			128,439	128,439	137,369
Net regulatory asset			333,599	333,599	300,008
Impairment loss - Nucla				37,067
Interest rate swap - unrealized gain
Regulatory assets and liabilities
Regulatory liabilities					8,576
Realized gain on interest rate swap settlement	$ 0
Interest rate swaps, realized gain
Regulatory assets and liabilities
Regulatory liabilities			3,861	3,861	4,215
Proceeds from settlement of interest rate swaps		$ 4,600
Deferred non-member electric sales
Regulatory assets and liabilities
Regulatory liabilities			82,006	82,006	82,006
Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Regulatory liabilities			42,572	$ 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			17,173	$ 17,173	18,098
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory assets and liabilities
Regulatory assets			84,287	84,287	86,005
Annual amortization expense				$ 2,300
Amortization period				47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets			49,857	$ 49,857	51,994
Annual amortization expense				$ 2,800
Amortization period				25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets			37,453	$ 37,453	38,227
Annual amortization expense				$ 1,000
Amortization period				44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets			143,088	$ 143,088	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			36,686	$ 36,686
Impairment loss - Nucla			37,100
Amortization period				3 years 3 months 18 days
Deferred impairment loss | Holcomb Expansion
Regulatory assets and liabilities
Regulatory assets			$ 93,494	$ 93,494	$ 93,494
Annual amortization expense				$ 4,700
Amortization period				20 years