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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2019
Fair Value Disclosures [Abstract]  
Fair value of financial instruments The estimated fair values of our financial instruments, all of which are categorized in Level 1 of the fair value hierarchy, as of March 31, 2019 and December 31, 2018 have been calculated based upon available market information, and were as follows: 
 
March 31, 2019
 
December 31, 2018
 
Carrying
Amount
 
Fair
Value
 
Carrying
Amount
 
Fair
Value
Senior notes
$
8,104

 
$
8,254

 
$
8,102

 
$
7,632