Fair Value (Tables)

3 Months Ended

Mar. 31, 2024

Fair Value Disclosures [Abstract]

Schedule of assets and liabilities measured at fair value on a recurring basis

The tables below detail assets and liabilities measured at fair value on a recurring basis at March 31, 2024 and December 31, 2023.

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	March 31, 2024		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	259,404	$	259,404	$	—	$	—
International equity trust	141,293		—		141,293		—
Corporate bonds and debt	64,861		—		64,772		89
US Treasury securities	42,814		42,814		—		—
Mortgage backed securities	56,607		—		56,607		—
Domestic mutual funds	92,626		92,626		—		—
Municipal bonds	303		—		303		—
Federal agency securities	8,231		—		8,231		—
International mutual funds	2,449		—		2,449		—
Non-US Gov't bonds & private placements	3,708		—		3,708		—
Other	7,002		7,002		—		—
Long-term investments:
International equity trust	46,098		—		46,098		—
Corporate bonds and debt	14,949		—		14,949		—
US Treasury securities	14,783		14,783		—		—
Mortgage backed securities	17,068		—		17,068		—
Domestic mutual funds	387,897		387,897		—		—
Treasury STRIPS	226,643		—		226,643		—
Non-US Gov't bonds & private placements	1,775		—		1,775		—
Other	602		602		—		—
Short-term investments: Treasury STRIPS	135,341		—		135,341		—
Natural gas swaps	10,381		—		10,381		—

    

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	December 31, 2023		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	234,979	$	234,979	$	—	$	—
International equity trust	134,911		—		134,911		—
Corporate bonds and debt	67,986		—		67,900		86
US Treasury securities	43,917		43,917		—		—
Mortgage backed securities	58,763		—		58,763		—
Domestic mutual funds	85,481		85,481		—		—
Municipal bonds	303		—		303		—
Federal agency securities	7,256		—		7,256		—
Non-US Gov't bonds & private placements	2,717		—		2,717		—
International mutual funds	2,012		—		2,012		—
Other	2,914		2,914		—		—
Long-term investments:
International equity trust	43,202		—		43,202		—
Corporate bonds and debt	14,151		—		14,151		—
US Treasury securities	17,243		17,243		—		—
Mortgage backed securities	15,024		—		15,024		—
Domestic mutual funds	378,387		378,387		—		—
Treasury STRIPS	220,765		—		220,765		—
Non-US Gov't bonds & private placements	1,568		—		1,568		—
Other	392		392		—		—
Short-term investments: Treasury STRIPS	143,931		—		143,931		—
Natural gas swaps	13,445		—		13,445		—

Schedule of estimated fair values of long-term debt, including current maturities

The estimated fair values of our long-term debt, including current maturities at March 31, 2024 and December 31, 2023 were as follows:

	2024				2023
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	11,966,572	$	10,233,768	$	12,096,552	$	10,638,749