LONG-TERM DEBT - Components (Details) $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) instrument	Dec. 31, 2014 USD ($)
Unsecured debt
Number of unsecured debt instruments \| instrument	2
Unsecured notes	$ 55,900
Long-term debt, components
Total debt	3,371,679	$ 3,242,470
Less debt issuance costs	(21,201)	(22,254)
Less debt discounts	(8,739)	(8,894)
Plus debt premiums	23,218	26,726
Total debt adjusted for discounts, premiums and debt issuance costs	3,364,957	3,238,048
Less current maturities	(91,419)	(92,802)
Long-term debt	3,273,538	3,145,246
3.66% to 8.08% CFC, due through 2028
Long-term debt, components
Total debt	$ 86,979	92,977
Interest rate terms
Stated interest rate, minimum (as a percent)	3.66%
Stated interest rate, maximum (as a percent)	8.08%
Average interest rate (as a percent)	5.96%
2.63% to 6.17% CoBank, ACB, due through 2042
Long-term debt, components
Total debt	$ 278,086	287,798
Interest rate terms
Stated interest rate, minimum (as a percent)	2.63%
Stated interest rate, maximum (as a percent)	6.17%
Average interest rate (as a percent)	4.35%
First Mortgage Bonds, Series 2014E-1, 3.70% due 2024
Long-term debt, components
Total debt	$ 250,000	250,000
Interest rate terms
Stated interest rate (as a percent)	3.70%
First Mortgage Bonds, Series 2014E-2, 4.70% due 2044
Long-term debt, components
Total debt	$ 250,000	250,000
Interest rate terms
Stated interest rate (as a percent)	4.70%
First Mortgage Bonds, Series 2010A, 6.00% due 2040
Long-term debt, components
Total debt	$ 500,000	500,000
Interest rate terms
Stated interest rate (as a percent)	6.00%
First Mortgage Obligation, Series 2014B, Tranche 1, 3.90%, due through 2033
Long-term debt, components
Total debt	$ 180,000	180,000
Interest rate terms
Stated interest rate (as a percent)	3.90%
First Mortgage Obligation Series 2014B, Tranche 2, 4.30%, due through 2039
Long-term debt, components
Total debt	$ 20,000	20,000
Interest rate terms
Stated interest rate (as a percent)	4.30%
First Mortgage Obligation Series 2014B, Tranche 3, 4.45%, due through 2045
Long-term debt, components
Total debt	$ 550,000	550,000
Interest rate terms
Stated interest rate (as a percent)	4.45%
First Mortgage Obligation, Series 2009C, Tranche 1, 6.00%, due through 2019
Long-term debt, components
Total debt	$ 108,571	135,714
Interest rate terms
Stated interest rate (as a percent)	6.00%
First Mortgage Obligation, Series 2009C, Tranche 2, 6.31%, due through 2021
Long-term debt, components
Total debt	$ 110,000	110,000
Interest rate terms
Stated interest rate (as a percent)	6.31%
Variable rate CFC, as determined by CFC, 2.90% average for 2015, due through 2026
Long-term debt, components
Total debt	$ 644	687
Interest rate terms
Average interest rate (as a percent)	2.90%
Variable rate CFC, LIBOR-based term loan, 1.48% average for 2015, due through 2049
Long-term debt, components
Total debt	$ 102,220	102,220
Variable rate CFC, LIBOR-based term loan, 1.48% average for 2015, due through 2049 \| LIBOR
Interest rate terms
Average interest rate (as a percent)	1.48%
Variable rate CoBank, ACB, LIBOR-based term loan, 1.74% average for 2015, due through 2044
Long-term debt, components
Total debt	$ 102,220	102,220
Variable rate CoBank, ACB, LIBOR-based term loan, 1.74% average for 2015, due through 2044 \| LIBOR
Interest rate terms
Average interest rate (as a percent)	1.74%
Revolving Credit Agreement
Long-term debt, components
Total debt	$ 271,000	50,000
Revolving Credit Agreement \| LIBOR
Interest rate terms
Average interest rate (as a percent)	1.25%
Pollution control revenue bonds, City of Gallup, NM, 5.00%, Series 2005, due through 2017
Long-term debt, components
Total debt	$ 10,815	15,840
Interest rate terms
Stated interest rate (as a percent)	5.00%
Pollution control revenue bonds, Moffat County, CO Variable Rate Demand Series 2009, due 2036
Long-term debt, components
Total debt	$ 46,800	46,800
Interest rate terms
Average interest rate (as a percent)	0.06%
Springerville certificates, Series A, 6.04%, due through 2018
Long-term debt, components
Total debt	$ 89,968	124,779
Interest rate terms
Stated interest rate (as a percent)	6.04%
Springerville certificates, Series B, 7.14%, due through 2033
Long-term debt, components
Total debt	$ 405,000	405,000
Interest rate terms
Stated interest rate (as a percent)	7.14%
Colowyo Bonds
Long-term debt, components
Total debt	$ 7,693	15,899
Interest rate terms
Stated interest rate (as a percent)	10.19%
Other
Long-term debt, components
Total debt	$ 1,683	$ 2,536

The number of unsecured debt instruments.

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Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
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Contractual interest rate for funds borrowed, under the debt agreement.

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Maximum contractual interest rate for funds borrowed, under the debt agreement.

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Minimum contractual interest rate for funds borrowed, under the debt agreement.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 30
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The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-SubTopic 210
-Section S99
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Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Weighted average interest rate of long-term debt outstanding.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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