ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands		9 Months Ended
	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2015
Regulatory assets and liabilities
Regulatory assets		$ 413,700	$ 415,081
Regulatory liabilities		92,572	45,000
Net regulatory asset		321,128	370,081
Deferred revenue
Regulatory assets and liabilities
Regulatory liabilities		45,000	45,000
Deferred revenue received in 2008
Regulatory assets and liabilities
Regulatory liabilities		10,000
Deferred revenue received in 2011
Regulatory assets and liabilities
Regulatory liabilities		35,000
Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Regulatory liabilities		47,572
KCEC | Deferred revenue from membership withdrawal
Regulatory assets and liabilities
Deferred membership withdrawal income	$ 47,600
Deferred income tax expense
Regulatory assets and liabilities
Regulatory assets		26,242	28,629
Deferred prepaid lease expense | Craig 3 Lease
Regulatory assets and liabilities
Regulatory assets		11,328	16,183
Annual amortization expense		6,500
Deferred prepaid lease expense | Springerville 3 Lease
Regulatory assets and liabilities
Regulatory assets		91,160	92,878
Annual amortization expense		$ 2,300
Amortization period		47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets		$ 58,404	60,541
Annual amortization expense		$ 2,800
Amortization period		25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets		$ 40,552	41,327
Annual amortization expense		$ 1,000
Amortization period		44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets		$ 168,973	175,444
Annual amortization expense		$ 8,600
Amortization period		21 years 4 months 24 days
Interest rate swaps
Regulatory assets and liabilities
Regulatory assets		$ 17,041
Other
Regulatory assets and liabilities
Regulatory assets			$ 79
Withdrawal Agreement | KCEC
Regulatory assets and liabilities
Net cash received from the early termination of membership	37,000
Early termination fees received for withdrawal from membership	49,500
Retirement of patronage capital	$ 12,500