ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Regulatory assets and liabilities
Regulatory assets	$ 416,447	$ 415,081
Regulatory liabilities	92,685	45,000
Net regulatory asset	323,762	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
Other
Regulatory assets and liabilities
Regulatory liabilities	113
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	28,067	28,629
Deferred prepaid lease expense | Craig 3 Lease
Regulatory assets and liabilities
Regulatory assets	12,947	16,183
Annual amortization expense	6,500
Deferred prepaid lease expense | Springerville 3 Lease
Regulatory assets and liabilities
Regulatory assets	91,732	92,878
Annual amortization expense	$ 2,300
Amortization period	47 years
Goodwill | TCP
Regulatory assets and liabilities
Regulatory assets	$ 59,117	60,541
Annual amortization expense	$ 2,800
Amortization period	25 years
Goodwill | Colowyo Coal
Regulatory assets and liabilities
Regulatory assets	$ 40,810	41,327
Annual amortization expense	$ 1,000
Amortization period	44 years
Deferred debt prepayment transaction costs
Regulatory assets and liabilities
Regulatory assets	$ 171,130	175,444
Annual amortization expense	$ 8,600
Amortization period	21 years 4 months 24 days
Interest rate swaps
Regulatory assets and liabilities
Regulatory assets	$ 12,644
Other
Regulatory assets and liabilities
Regulatory assets		$ 79