v3.26.1
Fair Value
 6 Months Ended
Jun. 30, 2026
Fair Value Disclosures [Abstract]  
Fair Value Fair Value.    Authoritative guidance regarding fair value measurements for financial and non-financial assets and liabilities defines fair value, establishes a framework for measuring fair value in accordance with generally accepted accounting principles, and expands disclosures about fair value measurements.
The guidance establishes a three-tier fair value hierarchy which prioritizes the inputs used in measuring fair value as follows:
•Level 1.  Quoted prices from active markets for identical assets or liabilities as of the reporting date. Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis. Quoted prices in active markets provide the most reliable evidence of fair value and are used to measure fair value whenever available. Level 1 primarily consists of financial instruments that are exchange-traded.

•Level 2.  Pricing inputs other than quoted prices in active markets included in Level 1, which are either directly or indirectly observable as of the reporting date. Level 2 includes financial instruments that are valued using models or other valuation methodologies. These models are primarily industry-standard models that consider various assumptions, including quoted forward prices for commodities, time value, volatility factors, and current market and contractual prices for the underlying instruments, as well as other relevant economic measures. Level 2 primarily consists of financial instruments that are non-exchange-traded but have significant observable inputs.

•Level 3.  Pricing inputs that include significant inputs which are generally less observable from objective sources. These inputs may include internally developed methodologies that result in management's best estimate of fair value. Level 3 financial instruments are those whose fair value is based on significant unobservable inputs.
As required by the guidance, assets and liabilities measured at fair value are based on one or more of the following three valuation techniques:
1.Market approach.    The market approach uses prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities (including a business) and deriving fair value based on these inputs.
2.Income approach.    The income approach uses valuation techniques to convert future amounts (for example, cash flows or earnings) to a single present amount (discounted). The measurement is based on the value indicated by current market expectations about those future amounts.

3.Cost approach.    The cost approach is based on the amount that currently would be required to replace the service capacity of an asset (often referred to as current replacement cost). This approach assumes that the fair value would not exceed what it would cost a market participant to acquire or construct a substitute asset of comparable utility, adjusted for obsolescence.
The tables below detail assets and liabilities measured at fair value on a recurring basis at June 30, 2026 and December 31, 2025.
 Fair Value Measurements at Reporting Date Using  
 Quoted Prices in
Active Markets for
Identical Assets
 Significant Other
Observable
Inputs
 Significant
Unobservable
Inputs
June 30, 2026(Level 1)(Level 2)(Level 3)
(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity$279,655 $279,655 $— $— 
Corporate bonds and debt119,822 — 119,642 180 
US Treasury securities85,160 85,160 — — 
Mortgage backed securities62,722 — 62,722 — 
Domestic mutual funds132,450 132,450 — — 
Municipal bonds3,764 — 3,764 — 
Federal agency securities8,093 — 8,093 — 
Non-US Gov't bonds & private placements19,756 — 19,756 — 
International mutual funds206,888 — 206,888 — 
Other15,137 15,137 
Long-term investments:
Corporate bonds and debt30,021 — 30,021 — 
US Treasury securities44,025 44,025 — — 
Mortgage backed securities24,810 — 24,810 — 
Domestic mutual funds434,624 434,624 — — 
Treasury STRIPS53,584 — 53,584 — 
Non-US Gov't bonds & private placements3,039 — 3,039 — 
International mutual funds55,829 — 55,829 — 
Other412 412 — — 
Short-term investments: Treasury STRIPS66,161 — 66,161 — 
Natural gas swaps15,176 — 15,176 — 
    
 Fair Value Measurements at Reporting Date Using  
 Quoted Prices in
Active Markets for
Identical Assets
 Significant Other
Observable
Inputs
 Significant
Unobservable
Inputs
December 31, 2025(Level 1)(Level 2)(Level 3)
(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity$254,191 $254,191 $— $— 
Corporate bonds and debt108,799 — 107,808 991 
US Treasury securities85,520 85,520 — — 
Mortgage backed securities65,814 — 65,814 — 
Domestic mutual funds109,307 109,307 — — 
Municipal bonds6,627 — 6,627 — 
Federal agency securities9,082 — 9,082 — 
Non-US Gov't bonds & private placements14,234 — 14,234 — 
International mutual funds180,112 — 180,112 — 
Other21,537 21,537 — — 
Long-term investments:
Corporate bonds and debt27,967 — 27,967 — 
US Treasury securities36,716 36,716 — — 
Mortgage backed securities28,996 — 28,996 — 
Domestic mutual funds430,593 430,593 — — 
Treasury STRIPS76,392 — 76,392 — 
Non-US Gov't bonds & private placements3,294 — 3,294 — 
International mutual funds48,935 — 48,935 — 
Other372 372 — — 
Short-term investments: Treasury STRIPS80,164 — 80,164 — 
Natural gas swaps10,642 — 10,642 — 

The Level 2 investments above may not be exchange traded. The fair value measurements for these investments are based on a market approach, including the use of observable inputs at or near the valuation date. Common inputs include reported trades and broker/dealer bid/ask prices.
The Level 3 investments above in corporate bonds and debt consist of investments in bank loans which are not exchange traded. Although these securities may be liquid and priced daily, their inputs are not observable.
The estimated fair values of our long-term debt, including current maturities at June 30, 2026 and December 31, 2025 were as follows:
20262025
Carrying
Value		Fair
Value		Carrying
Value		Fair
Value
(in thousands)
Long-term debt$12,585,504 $10,920,819 $12,566,059 $11,008,470 
The estimated fair value of long-term debt is classified as Level 2 and is estimated based on observed or quoted market prices for the same or similar issues or on current rates offered to us for debt of similar maturities. The primary sources of our long-term debt consist of first mortgage bonds, pollution control revenue bonds and long-term debt issued by the Federal Financing Bank that is guaranteed by the Rural Utilities Service or the U.S. Department of Energy. The valuations for the first mortgage bonds and the pollution control revenue bonds were obtained from a third party data reporting service, and are based on secondary market trading of our debt. Valuations for debt issued by the Federal
Financing Bank are based on U.S. Treasury rates as of June 30, 2026 and December 31, 2025 plus an applicable spread, which reflects our borrowing rate for new loans of this type from the Federal Financing Bank.
For cash and cash equivalents and receivables, the carrying amount approximates fair value because of the short-term maturity of those instruments.