Fair Value (Tables)

12 Months Ended

Dec. 31, 2022

Fair Value Disclosures [Abstract]

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

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

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in		Significant Other		Significant
			Active Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
	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

	2022				2021
	Carrying		Fair		Carrying		Fair
	Value		Value		Value		Value
	(in thousands)
Long-term debt	$	11,940,359	$	10,194,954	$	10,915,054	$	12,741,046