ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Regulatory assets and liabilities
Regulatory assets	$ 390,296	$ 395,615
Regulatory liabilities	87,575	95,512
Net regulatory asset	302,721	300,103
Interest rate swaps
Regulatory assets and liabilities
Regulatory liabilities	14,202	12,140
Deferred revenue
Regulatory assets and liabilities
Regulatory liabilities	28,300	35,800
Deferred revenue received in 2008
Regulatory assets and liabilities
Regulatory liabilities	10,000
Deferred revenue expected to be recognized in 2017	10,000
Deferred revenue received in 2011
Regulatory assets and liabilities
Regulatory liabilities	35,000
Deferred revenue expected to be recognized in 2017	20,000
Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Regulatory liabilities	45,073	47,572
Deferred revenue expected to be recognized in 2017	10,000
KCEC | Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Deferred membership withdrawal income			$ 47,600
Non-member electric sales | Deferred revenue
Regulatory assets and liabilities
Deferred revenue recognized	7,500	9,200
Other Income | Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Deferred revenue recognized	2,500
Deferred income tax expense
Regulatory assets and liabilities
Regulatory assets	30,517	30,517
Deferred prepaid lease expense | Craig 3 Lease
Regulatory assets and liabilities
Regulatory assets	8,091	9,710
Annual amortization expense	6,500
Deferred prepaid lease expense | Springerville 3 Lease
Regulatory assets and liabilities
Regulatory assets	90,014	90,587
Annual amortization expense	$ 2,300
Amortization period	47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets	$ 56,980	57,692
Annual amortization expense	$ 2,800
Amortization period	25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets	$ 40,035	40,294
Annual amortization expense	$ 1,000
Amortization period	44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets	$ 164,659	$ 166,815
Annual amortization expense	$ 8,600
Amortization period	21 years 4 months 24 days