Fair Value (Tables)

12 Months Ended

Dec. 31, 2021

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, 2021		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	$	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		—

	Fair Value Measurements at Reporting Date Using
	December 31, 2020		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	$	198,325	$	198,325	$	—	$	—
International equity trust	$	120,645	—		120,645		—
Corporate bonds and debt	$	98,129	—		97,788		341
US Treasury securities	$	46,963	46,963		—		—
Mortgage backed securities	$	45,039	—		45,039		—
Domestic mutual funds	$	70,813	70,813		—		—
Municipal bonds	$	1,362	—		1,362		—
Federal agency securities	$	6,054	—		6,054		—
Other	$	10,851	7,720		3,131		—
Long-term investments:
International equity trust	$	31,378	—		31,378		—
Corporate bonds and debt	$	29,870	—		29,661		209
US Treasury securities	$	7,437	7,437		—		—
Mortgage backed securities	$	11,432	—		11,432		—
Domestic mutual funds	$	224,536	224,536		—		—
Federal agency securities	$	537	—		537		—
Treasury STRIPS	$	209,165	—		209,165		—
Other	$	3,710	3,710		—		—
Natural gas swaps	$	10,248	—		10,248		—

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, 2021 and 2020 were as follows:

	2021				2020
	(in thousands)
	Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	10,915,054	$	12,741,046	$	10,619,826	$	13,161,146