Fair Value (Tables)

6 Months Ended

Jun. 30, 2020

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, 2020 and December 31, 2019.

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	June 30, 2020		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	162,846	$	162,846	$	—	$	—
International equity trust	93,046		—		93,046		—
Corporate bonds and debt	88,181		—		88,181		—
US Treasury securities	41,699		41,699		—		—
Mortgage backed securities	62,873		—		62,873		—
Domestic mutual funds	50,577		50,577		—		—
Municipal bonds	1,398		—		1,398		—
Federal agency securities	1,348		—		1,348		—
Non-US Gov't bonds & private placements	1,377		—		1,377		—
Other	6,003		6,003		—		—
Long-term investments:
International equity trust	23,044		—		23,044		—
Corporate bonds and debt	27,249		—		27,249		—
US Treasury securities	6,408		6,408		—		—
Mortgage backed securities	12,884		—		12,884		—
Domestic mutual funds	124,595		124,595		—		—
Federal agency securities	1,161		—		1,161		—
Treasury STRIPS	133,392		—		133,392		—
Other	2,267		2,267		—		—
Natural gas swaps	31,929		—		31,929		—

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
	December 31, 2019		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	179,346	$	179,346	$	—	$	—
International equity trust	96,204		—		96,204		—
Corporate bonds and debt	63,849		—		63,849		—
US Treasury securities	45,522		45,522		—		—
Mortgage backed securities	62,400		—		62,400		—
Domestic mutual funds	55,522		55,522		—		—
Municipal bonds	1,189		—		1,189		—
Federal agency securities	2,586		—		2,586		—
Other	4,721		4,450		271		—
Long-term investments:
International equity trust	23,161		—		23,161		—
Corporate bonds and debt	20,395		—		20,395		—
US Treasury securities	9,257		9,257		—		—
Mortgage backed securities	12,867		—		12,867		—
Domestic mutual funds	126,380		126,380		—		—
Federal agency securities	1,082		—		1,082		—
Treasury STRIPS	59,816		—		59,816		—
Other	1,906		1,906		—		—
Natural gas swaps	32,256		—		32,256		—

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, 2020 and December 31, 2019 were as follows:

	2020				2019
	Carrying Value		Fair Value		Carrying Value		Fair Value
	​(in thousands)
Long-term debt	$	10,105,932	$	12,893,268	$	9,726,428	$	11,180,658