Our flight equipment and other long-lived assets have a recorded value of $21.9 billion at December 31, 2014 . This value is based on various factors, including the assets' estimated useful lives and salvage values. We record impairment losses on flight equipment and other long-lived assets used in operations when events and circumstances indicate the assets may be impaired and the estimated future cash flows generated by those assets are less than their carrying amounts. Factors which could cause impairment include, but are not limited to, (1) a decision to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) significant changes in projected cash flows, (4) permanent and significant declines in fleet fair values and (5) changes to the regulatory environment. For long-lived assets held for sale, we discontinue depreciation and record impairment losses when the carrying amount of these assets is greater than the fair value less the cost to sell.  

 

To determine whether impairments exist for aircraft used in operations, we group assets at the fleet-type level (the lowest level for which there are identifiable cash flows) and then estimate future cash flows based on projections of capacity, passenger mile yield, fuel costs, labor costs and other relevant factors. If an impairment occurs, the impairment loss recognized is the amount by which the fleet's carrying amount exceeds its estimated fair value. We estimate aircraft fair values using published sources, appraisals and bids received from third parties, as available.  

 

Income Tax Valuation Allowance  

 

We periodically assess whether it is more likely than not that we will generate sufficient taxable income to realize our deferred income tax assets. We establish valuation allowances if it is not likely we will realize our deferred income tax assets. In making this determination, we consider all available positive and negative evidence and make certain assumptions. We consider, among other things, projected future taxable income, scheduled reversals of deferred tax liabilities, the overall business environment, our historical financial results, and tax planning strategies. In evaluating the likelihood of utilizing our net deferred income tax assets, the significant factors that we consider include (1) our recent history of profitability, (2) growth in the U.S. and global economies, (3) forecast of airline revenue trends, (4) estimate of future fuel prices and (5) future impact of taxable temporary differences.  

 

We recorded a full valuation allowance in 2004 due to our cumulative loss position at that time. During 2013 , after considering all positive and negative evidence, we concluded that our deferred income tax assets are more likely than not to be realized. Accordingly, we released substantially all of the valuation allowance against our net deferred income tax assets. We recognized an $8.0 billion benefit in our provision for income taxes, primarily related to the valuation allowance release.  

 

At the end of 2014 our net deferred tax asset balance was $7.6 billion , against which