Fair Value (Tables)

6 Months Ended

Jun. 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 June 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
	June 30, 2021		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	218,215	$	218,215	$	—	$	—
International equity trust	126,812		—		126,812		—
Corporate bonds and debt	90,406		—		89,917		489
US Treasury securities	59,569		59,569		—		—
Mortgage backed securities	42,658		—		42,658		—
Domestic mutual funds	76,373		76,373		—		—
Municipal bonds	1,127		—		1,127		—
Federal agency securities	11,149		—		11,149		—
Non-US Gov't bonds & private placements	2,586		—		2,586		—
Other	12,564		12,564		—		—
Long-term investments:
International equity trust	33,003		—		33,003		—
Corporate bonds and debt	24,636		—		24,322		314
US Treasury securities	14,304		14,304		—		—
Mortgage backed securities	12,039		—		12,039		—
Domestic mutual funds	244,150		244,150		—		—
Federal agency securities	361		—		361		—
Treasury STRIPS	285,562		—		285,562		—
Other	7,322		7,322		—		—
Natural gas swaps	36,723		—		36,723		—

	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 June 30, 2021 and December 31, 2020 were as follows:

	2021				2020
	Carrying Value		Fair Value		Carrying Value		Fair Value
	(in thousands)
Long-term debt	$	11,003,032	$	12,920,247	$	10,619,826	$	13,161,146