Fair Value (Tables)

9 Months Ended

Sep. 30, 2022

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, 2022 and December 31, 2021.

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	September 30, 2022		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	189,706	$	189,706	$	—	$	—
International equity trust	97,294		—		97,294		—
Corporate bonds and debt	63,283		—		63,283		—
US Treasury securities	44,191		44,191		—		—
Mortgage backed securities	38,779		—		38,779		—
Domestic mutual funds	52,740		52,740		—		—
Federal agency securities	2,958		—		2,958		—
Non-US Gov't bonds & private placements	2,842		—		2,842		—
Other	11,659		11,659		—		—
Long-term investments:
International equity trust	29,375		—		29,375		—
Corporate bonds and debt	13,444		—		13,346		98
US Treasury securities	11,141		11,141		—		—
Mortgage backed securities	12,725		—		12,725		—
Domestic mutual funds	264,413		264,413		—		—
Treasury STRIPS	297,346		—		297,346		—
Non-US Gov't bonds & private placements	1,886		—		1,886		—
Other	81		81		—		—
Short-term investments: Treasury STRIPS	52,061		—		52,061		—
Natural gas swaps	192,026		—		192,026		—

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	December 31, 2021		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	249,999	$	249,999	$	—	$	—
International equity trust	140,718		—		140,718		—
Corporate bonds and debt	72,936		—		72,369		567
US Treasury securities	53,321		53,321		—		—
Mortgage backed securities	40,460		—		40,460		—
Domestic mutual funds	75,384		75,384		—		—
Municipal bonds	1,133		—		1,133		—
Federal agency securities	9,608		—		9,608		—
Other	16,351		13,623		2,728		—
Long-term investments:
International equity trust	35,873		—		35,873		—
Corporate bonds and debt	14,022		—		12,656		1,366
US Treasury securities	15,259		15,259		—		—
Mortgage backed securities	12,021		—		12,021		—
Domestic mutual funds	277,937		277,937		—		—
Federal agency securities	257		—		257		—
Treasury STRIPS	350,532		—		350,532		—
Other	5,478		5,478		—		—
Natural gas swaps	63,994		—		63,994		—

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, 2022 and December 31, 2021 were as follows:

	2022				2021
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	11,356,651	$	9,585,512	$	10,915,054	$	12,741,046