Property, Plant and Equipment

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Net property, plant and equipment is composed of the following:


	Depreciable Lives	As of December 31,
		2020		2019
		(Dollars in millions)
Land	N/A	$	320	336
Fiber conduit and other outside plant (1)	15-45 years	6,186		5,226
Central office and other network electronics (2)	7-10 years	3,388		2,687
Support assets (3)	3-30 years	2,722		2,419
Construction-in-progress (4)	N/A	720		1,093
Gross property, plant and equipment		13,336		11,761
Accumulated depreciation		(2,818)		(1,825)
Net property, plant and equipment		$	10,518	9,936

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(1)Fiber, conduit and other outside plant consists of fiber and metallic cable, conduit, poles and other supporting structures.

(2)Central office and other network electronics consists of circuit and packet switches, routers, transmission electronics and electronics providing service to customers.

(3)Support assets consist of buildings, data centers, computers and other administrative and support equipment.

(4)Construction in progress includes construction and property of the aforementioned categories that has not been placed in service as it is still under construction.

Depreciation expense was $851 million, $804 million and $906 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Asset Retirement Obligations

As of December 31, 2020 and 2019, our asset retirement obligations consisted primarily of restoration requirements for leased facilities. We recognize our estimate of the fair value of our asset retirement obligations in the period incurred in other long-term liabilities. The fair value of the asset retirement obligation is also capitalized as property, plant and equipment and then depreciated over the estimated remaining useful life of the associated asset.

The following table provides asset retirement obligation activity:


	Years Ended December 31,
	2020		2019	2018
	(Dollars in millions)
Balance at beginning of period	$	113	105	45
Accretion expense	6		5	5
Purchase price adjustments (1)	—		—	58
Liabilities settled	(7)		(12)	(13)
Revision in estimated cash flows	10		15	10
Balance at end of period	$	122	113	105

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(1)These liabilities relate to purchase price adjustments that occurred during 2018 from Lumen's acquisition of us.