Property, plant and equipment, net at December 31, 2020 and 2019 are as follows:



	Estimated
($ in millions)	Useful Lives	2020		2019

Land	N/A	$	212	$	217
Buildings and leasehold improvements	40 years		2,139		2,171
General support	5 to 15 years		1,643		1,624
Central office/electronic circuit equipment	5 to 8 years		8,270		7,968
Poles	30 years		1,371		1,274
Cable, fiber and wire	15 to 25 years		11,883		11,312
Conduit	50 years		1,619		1,608
Construction work in progress			558		378
Property, plant and equipment			27,695		26,552
Less: Accumulated depreciation			(14,764)		(13,589)
Property, plant and equipment, net		$	12,931	$	12,963

As of December 31, 2019, $1,049 million of fixed assets were reclassified to assets held for sale in relation to the planned sale of the Northwest Operations (see Note 7). Effective with the designation of the Northwest Operations

as held-for-sale on May 28, 2019, we discontinued recording depreciation on Property, Plant and Equipment and finite-lived intangible assets of this business as required by GAAP.

Property, plant, and equipment includes approximately $143 million, $167 million, and $152 million of fixed assets recognized under capital leases as of December 31, 2020, 2019 and 2018, respectively.

During 2020, we sold certain properties consisting of land and buildings for approximately $27 million in cash. The aggregate carrying value of the properties was approximately $37 million, resulting in a loss on the sale of $10 million, which, given our composite group method of accounting for depreciation, was recognized against “Accumulated Depreciation” in our consolidated balance sheet.

In 2017 and 2018, we sold certain properties subject to leaseback, generating $106 million in net proceeds. In connection with the adoption of ASC 842, the $15 million ($11 million net of tax) unamortized deferred gains resulting from these transactions were recognized directly to opening accumulated deficit as of January 1, 2019.

In January 2019, we closed the sale of certain wireless towers for approximately $76 million in cash. The aggregate carrying value of the towers was approximately $1 million, resulting in a gain on the sale of $75 million which, given our composite group method of accounting for depreciation, was recognized against “Accumulated Depreciation” in our consolidated balance sheet during 2020.

Depreciation expense is principally based on the composite group method. Depreciation expense was as follows:



	For the year ended December 31,
($ in millions)	2020		2019		2018

Depreciation expense	$	1,255	$	1,335	$	1,385