Note 9 Property, Plant and Equipment

Property, plant and equipment in service and under construction, and related accumulated depreciation and amortization, as of December 31, 2017 and 2016, were as follows:

December 31,	Useful Lives (Years)	2017		2016
(Dollars in millions)
Land	N/A	$	36	$	35
Buildings	20		297		297
Leasehold and land improvements	1-30		1,178		1,153
Cell site equipment	7-25		3,411		3,383
Switching equipment	5-8		988		976
Office furniture and equipment	3-5		389		420
Other operating assets and equipment	3-5		57		53
System development	1-7		1,060		1,217
Work in process	N/A		212		178
Total property, plant and equipment, gross			7,628		7,712
Accumulated depreciation and amortization			(5,308)		(5,242)
Total property, plant and equipment, net		$	2,320	$	2,470

Depreciation and amortization expense totaled $604 million, $607 million and $596 million in 2017, 2016 and 2015, respectively. In 2017, 2016 and 2015, (Gain) loss on asset disposals, net included charges of $17 million, $22 million and $16 million, respectively, related to disposals of assets, trade-ins of older assets for replacement assets and other retirements of assets from service in the normal course of business.