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Fair Value (Tables)
 12 Months Ended
Dec. 31, 2019
Fair Value Disclosures [Abstract]  
Schedule of assets and liabilities measured at fair value on a recurring basis
Fair Value Measurements at Reporting Date Using
December 31, 2019Quoted Prices in
Active Markets for
Identical Assets
(Level 1)		Significant Other
Observable Inputs
(Level 2)		Significant
Unobservable Inputs
(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  —  

Fair Value Measurements at Reporting Date Using
December 31, 2018Quoted Prices in
Active Markets for
Identical Assets
(Level 1)		Significant Other
Observable
Inputs
(Level 2)		Significant
Unobservable
Inputs
(Level 3)
(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity$136,196  $136,196  $—  $—  
International equity trust$76,852  —  76,852  —  
Corporate bonds and debt$51,356  —  48,853  2,503  
US Treasury securities$47,712  47,712  —  —  
Mortgage backed securities$56,004  —  56,004  —  
Domestic mutual funds$43,359  43,359  —  —  
Municipal bonds$278  —  278  —  
Federal agency securities$6,066  —  6,066  —  
Other$2,995  2,031  964  —  
Long-term investments:
International equity trust$17,382  —  17,382  —  
Corporate bonds and debt$12,571  —  11,366  1,205  
US Treasury securities$12,062  12,062  —  —  
Mortgage backed securities$11,517  —  11,517  —  
Domestic mutual funds$94,494  94,494  —  —  
Federal agency securities$941  —  941  —  
Treasury STRIPS$14,113  —  14,113  —  
Other$1,045  1,045  —  —  
Natural gas swaps$13,154  —  13,154  —  
Schedule of changes in Level 3 assets measured at fair value on a recurring basis
The following table presents the changes in Level 3 assets measured at fair value on a recurring basis at December 31, 2019 and December 31, 2018.
Year Ended December 31, 2019
(dollars in
thousands)
Balance at December 31, 2018$3,708  
Total gains or losses (realized/unrealized):
   Included in earnings (or changes in net assets)94  
Liquidations(3,802) 
Balance at December 31, 2019$—  
Year Ended December 31, 2018
(dollars in
thousands)
Balance at December 31, 2017$—  
Transfers to Level 34,997  
Total gains or losses (realized/unrealized):
   Included in earnings (or changes in net assets)(1,289) 
Balance at December 31, 2018$3,708  
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 December 31, 2019 and 2018 were as follows (in thousands):
20192018
Carrying
Value		Fair
Value		Carrying
Value		Fair
Value
Long-term debt$9,726,428  $11,180,658  $9,347,307  $9,837,254