Fair Value (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

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

	Fair Value Measurements at Reporting Date Using
	December 31, 2023		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (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		—

	Fair Value Measurements at Reporting Date Using
	December 31, 2022		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	204,129	$	204,129	$	—	$	—
International equity trust	$	111,266	—		111,266		—
Corporate bonds and debt	$	60,806	—		60,788		18
US Treasury securities	$	49,775	49,775		—		—
Mortgage backed securities	$	41,210	—		41,210		—
Domestic mutual funds	$	57,348	57,348		—		—
Federal agency securities	$	2,037	—		2,037		—
Non-US Gov't bonds & private placements	$	2,890	—		2,890		—
International mutual funds	$	653			653
Other	$	10,602	10,602		—		—
Long-term investments:
International equity trust	$	33,606	—		33,606		—
Corporate bonds and debt	$	10,473	—		10,473		—
US Treasury securities	$	15,488	15,488		—		—
Mortgage backed securities	$	12,113	—		12,113		—
Domestic mutual funds	$	302,302	302,302		—		—
Treasury STRIPS	$	293,281	—		293,281		—
Non-US Gov't bonds & private placements	$	1,976	—		1,976		—
Other	$	240	240		—		—
Short-term investments: Treasury STRIPS	$	61,702			61,702
Natural gas swaps	$	131,804	—		131,804		—

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 December 31, 2023 and 2022 were as follows:

	2023				2022
	(in thousands)
	Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	12,096,552	$	10,638,749	$	11,940,359	$	10,194,954