Plant and Depreciation
We record plant, property and equipment at cost. Plant, property and equipment of wireline and wireless operations are generally depreciated on a straight-line basis.
Leasehold improvements are amortized over the shorter of the estimated life of the improvement or the remaining term of the related lease, calculated from the time the asset was placed in service.
When the depreciable assets of our wireline and wireless operations are retired or otherwise disposed of, the related cost and accumulated depreciation are deducted from the plant accounts, and any gains or losses on disposition are recognized in income.
We capitalize and depreciate network software purchased or developed along with related plant assets. We also capitalize interest associated with the acquisition or construction of network-related assets. Capitalized interest is reported as a reduction in interest expense and depreciated as part of the cost of the network-related assets.
In connection with our ongoing review of the estimated remaining average useful lives of plant, property and equipment at our local telephone operations, we determined that there were no changes necessary for average useful lives for 2013, 2012 and 2011. In connection with our ongoing review of the estimated remaining average useful lives of plant, property and equipment at our wireless operations, we determined that changes were necessary to the remaining estimated useful lives as a result of technology upgrades, enhancements, and planned retirements. These changes resulted in an increase in depreciation expense of $0.4 billion in 2011. While the timing and extent of current deployment plans are subject to ongoing analysis and modification, we believe the current estimates of useful lives are reasonable.