LONG-TERM DEBT - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Long-term debt, components
Total debt	$ 3,137,534	$ 2,981,481
Less debt issuance costs	(19,723)	(21,481)
Less debt discounts	(8,678)	(8,960)
Plus debt premiums	10,896	11,843
Total debt adjusted for debt issuance costs, discounts and premiums	3,120,029	2,962,883
Less current maturities	(223,523)	(92,920)
Long-term debt	2,896,506	2,869,963
2.32% to 6.44% CFC, due through 2050
Long-term debt, components
Total debt	$ 177,260	85,855
2.32% to 6.44% CFC, due through 2050 | Minimum
Interest rate terms
Stated interest rate (as a percent)	2.32%
2.32% to 6.44% CFC, due through 2050 | Maximum
Interest rate terms
Stated interest rate (as a percent)	6.44%
2.63% to 4.43% CoBank, ACB, due through 2042
Long-term debt, components
Total debt	$ 159,736	174,985
2.63% to 4.43% CoBank, ACB, due through 2042 | Minimum
Interest rate terms
Stated interest rate (as a percent)	2.63%
2.63% to 4.43% CoBank, ACB, due through 2042 | Maximum
Interest rate terms
Stated interest rate (as a percent)	4.43%
First Mortgage Obligations, Series 2017A, Tranche 1, 3.34%, due through 2029
Long-term debt, components
Total debt	$ 60,000	60,000
Interest rate terms
Stated interest rate (as a percent)	3.34%
First Mortgage Obligations, Series 2017A, Tranche 2, 3.39%, due through 2029
Long-term debt, components
Total debt	$ 60,000	60,000
Interest rate terms
Stated interest rate (as a percent)	3.39%
First Mortgage Bonds, Series 2016A, 4.25% due 2046
Long-term debt, components
Total debt	$ 228,783	228,783
Interest rate terms
Stated interest rate (as a percent)	4.25%
First Mortgage Bonds, Series 2014E-1, 3.70% due 2024
Long-term debt, components
Total debt	$ 128,002	128,002
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	$ 499,805	499,805
Interest rate terms
Stated interest rate (as a percent)	6.00%
First Mortgage Obligations, 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 Obligations, 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 Obligations, 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%
Variable rate CFC, SOFR-based term loans, due through 2049
Long-term debt, components
Total debt	$ 152,220	152,220
Variable rate CoBank, ACB, SOFR-based term loans, due through 2044
Long-term debt, components
Total debt	273,925	296,430
Syndicated variable rate, SOFR-based term loan due 2025
Long-term debt, components
Total debt	150,000	0
Moffat County, CO, 2.90% term rate through October 2027, Series 2009, due 2036
Long-term debt, components
Total debt	$ 46,800	46,800
Interest rate terms
Stated interest rate (as a percent)	2.90%
Series B, 7.14%, due through 2033
Long-term debt, components
Total debt	$ 200,503	248,601
Interest rate terms
Stated interest rate (as a percent)	7.14%
New Horizon Mine remaining land installment payments
Long-term debt, components
Total debt	$ 500	$ 0