Fair Value (Tables)

9 Months Ended

Sep. 30, 2021

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, 2021 and December 31, 2020.

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	September 30, 2021		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	225,255	$	225,255	$	—	$	—
International equity trust	142,018		—		142,018		—
Corporate bonds and debt	78,874		—		78,306		568
US Treasury securities	54,212		54,212		—		—
Mortgage backed securities	33,712		—		33,712		—
Domestic mutual funds	74,861		74,861		—		—
Municipal bonds	1,130		—		1,130		—
Federal agency securities	9,792		—		9,792		—
Non-US Gov't bonds & private placements	2,696		—		2,696		—
Other	13,408		13,408		—		—
Long-term investments:
International equity trust	35,015		—		35,015		—
Corporate bonds and debt	15,590		—		15,272		318
US Treasury securities	13,440		13,440		—		—
Mortgage backed securities	11,582		—		11,582		—
Domestic mutual funds	271,354		271,354		—		—
Federal agency securities	290		—		290		—
Treasury STRIPS	325,398		—		325,398		—
Other	6,035		6,035		—		—
Natural gas swaps	93,930		—		93,930		—

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	December 31, 2020		(Level 1)		(Level 2)		(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 September 30, 2021 and December 31, 2020 were as follows:

	2021				2020
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	10,700,098	$	12,568,831	$	10,619,826	$	13,161,146