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Fair Value Disclosure (Tables)
 6 Months Ended
Jun. 30, 2021
Fair Value Disclosures [Abstract]  
Schedule of carrying amounts and estimated fair values of long-term debt, excluding capital lease obligations, and input level to determine fair values
The following table presents the carrying amounts and estimated fair values of our long-term debt, excluding finance lease and other obligations, as well as the input level used to determine the fair values indicated below:
  June 30, 2021December 31, 2020
 Input
Level		Carrying
Amount		Fair
Value		Carrying
Amount		Fair
Value
  (Dollars in millions)
Liabilities—Long-term debt (excluding finance lease and other obligations)2$3,102 3,304 3,328 3,532