Fair Value (Tables)

3 Months Ended

Mar. 31, 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 March 31, 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
	March 31, 2022		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	237,403	$	237,403	$	—	$	—
International equity trust	126,621		—		126,621		—
Corporate bonds and debt	47,987		—		46,718		1,269
US Treasury securities	102,406		102,406		—		—
Mortgage backed securities	22,192		—		22,192		—
Domestic mutual funds	68,348		68,348		—		—
Municipal bonds	809		—		809		—
Federal agency securities	2,268		—		2,268		—
Other	7,213		7,213		—		—
Long-term investments:
International equity trust	36,959		—		36,959		—
Corporate bonds and debt	4,129		—		2,938		1,191
US Treasury securities	40,382		40,382		—		—
Mortgage backed securities	—		—		—		—
Domestic mutual funds	271,155		271,155		—		—
Federal agency securities	—		—		—		—
Treasury STRIPS	350,275		—		350,275		—
Other	(79)		(79)		—		—
Natural gas swaps	173,946		—		173,946		—

	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 March 31, 2022 and December 31, 2021 were as follows:

	2022				2021
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	10,830,072	$	11,338,418	$	10,915,054	$	12,741,046