Fair Value (Tables)

3 Months Ended

Mar. 31, 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 March 31, 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
	March 31, 2020		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	134,999	$	134,999	$	—	$	—
International equity trust	$	78,208	—		78,208		—
Corporate bonds and debt	$	65,999	—		65,999		—
US Treasury securities	$	51,244	51,244		—		—
Mortgage backed securities	$	64,729	—		64,729		—
Domestic mutual funds	$	38,564	38,564		—		—
Municipal bonds	$	1,302	—		1,302		—
Federal agency securities	$	3,499	—		3,499		—
Non-US Gov't bonds & private placements	$	242	—		242		—
Other	$	6,034	6,034		—		—
Long-term investments:
International equity trust	$	18,257	—		18,257		—
Corporate bonds and debt	$	19,744	—		19,744		—
US Treasury securities	$	8,932	8,932		—		—
Mortgage backed securities	$	15,305	—		15,305		—
Domestic mutual funds	$	130,940	130,940		—		—
Federal agency securities	$	867	—		867		—
Treasury STRIPS	$	111,297	—		111,297		—
Other	$	2,070	2,070		—		—
Natural gas swaps	$	36,067	—		36,067		—

	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 March 31, 2020 and December 31, 2019 were as follows (in thousands):

	2020				2019
	Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	10,085,706	$	12,749,538	$	9,726,428	$	11,180,658