Fair Value (Tables)

9 Months Ended

Sep. 30, 2023

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 September 30, 2023 and December 31, 2022.

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	September 30, 2023		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	210,135	$	210,135	$	—	$	—
International equity trust	121,884		—		121,884		—
Corporate bonds and debt	68,052		—		68,013		39
US Treasury securities	43,616		43,616		—		—
Mortgage backed securities	42,443		—		42,443		—
Domestic mutual funds	71,496		71,496		—		—
Municipal bonds	282		—		282		—
Federal agency securities	8,199		—		8,199		—
International mutual funds	861		—		861		—
Non-US Gov't bonds & private placements	2,483		—		2,483		—
Other	7,048		7,048		—		—
Long-term investments:
International equity trust	37,721		—		37,721		—
Corporate bonds and debt	12,523		—		12,523		—
US Treasury securities	15,056		15,056		—		—
Mortgage backed securities	14,288		—		14,288		—
Domestic mutual funds	338,327		338,327		—		—
Treasury STRIPS	246,911		—		246,911		—
Non-US Gov't bonds & private placements	1,377		—		1,377		—
Other	511		511		—		—
Short-term investments: Treasury STRIPS	124,919		—		124,919		—
Natural gas swaps	47,054		—		47,054		—

    

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	December 31, 2022		(Level 1)		(Level 2)		(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 September 30, 2023 and December 31, 2022 were as follows:

	2023				2022
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	11,779,108	$	9,365,972	$	11,940,359	$	10,194,954