Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Allowance for uncollectible accounts	$ 12	$ 12
Allowance for obsolescence	102	113
Property and Equipment, Net:
Accumulated depreciation and amortization	15,823	14,097
Other Assets:
Accumulated amortization	$ 862	$ 845
Stockholders' Equity:
Common stock, par value (usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (shares)	1,500,000,000	1,500,000,000
Common stock, shares issued (shares)	688,136,306	714,674,160
Treasury stock, at cost (shares)	8,191,831	7,476,181

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The amount of the valuation account as of the balance sheet date which reduces the carrying amount of airline related inventory (including expendable merchandise, goods, commodities, or supplies to be used primarily in air transport of passengers or freight) to net realizable value; takes into consideration such factors as market value, excessive quantities based on expected sales, technological obsolescence, and shrinkage.

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