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Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2021
Fair Value Disclosures [Abstract]  
Fair value of financial instruments The estimated fair values of our other financial instruments, all of which are categorized in Level 1 of the fair value hierarchy, as of September 30, 2021 and December 31, 2020 have been calculated based upon available market information, and were as follows: 
 September 30, 2021December 31, 2020
 Carrying
Amount
Fair
Value
Carrying
Amount
Fair
Value
Senior notes$6,713 $7,084 $6,965 $7,470