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10. Fair Value Measurements (Tables)
 6 Months Ended
Jun. 30, 2019
Fair Value Disclosures [Abstract]  
Financial instruments
    June 30, 2019       December 31, 2018    
    Carrying Value     Estimated Fair Value     Carrying Value     Estimated Fair Value  
Assets:                        
Trade receivables, net   $ 524     $ 524     $ 429     $ 429  
Equipment financing receivables     457       457       251       251  
Liabilities:                                
Finance lease obligations   $ 127     $ 127     $ 144     $ 144  
Notes payable     7       7       56       56