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Fair Value Measurements (Tables)
 6 Months Ended
Jun. 30, 2017
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 June 30, 2017 and December 31, 2016 have been calculated based upon available market information, and were as follows: 
 
June 30, 2017
 
December 31, 2016
 
Carrying
Amount
 
Fair
Value
 
Carrying
Amount
 
Fair
Value
Senior notes
$
5,770

 
$
6,058

 
$
5,506

 
$
5,715