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	$	848	867
Fiber, conduit and other outside plant(1)	15-45 years	26,522		24,666
Central office and other network electronics(2)	3-10 years	20,692		19,608
Support assets(3)	3-30 years	8,261		7,984
Construction in progress(4)	N/A	1,611		2,300
Gross property, plant and equipment		57,934		55,425
Accumulated depreciation		(31,596)		(29,346)
Net property, plant and equipment		$	26,338	26,079

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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, cable landing stations, data centers, computers and other administrative and support equipment.

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

We recorded depreciation expense of $3.0 billion, $3.1 billion and $3.3 billion for the years ended December 31, 2020, 2019 and 2018, respectively.

Asset Retirement Obligations

At December 31, 2020, our asset retirement obligations balance was primarily related to estimated future costs of removing equipment from leased properties and estimated future costs of properly disposing of asbestos and other hazardous materials upon remodeling or demolishing buildings. Asset retirement obligations are included in other long-term liabilities on our consolidated balance sheets.

Our fair value estimates were determined using the discounted cash flow method.

The following table provides asset retirement obligation activity:


	Years Ended December 31,
	2020		2019	2018
	(Dollars in millions)
Balance at beginning of year	$	197	190	115
Accretion expense	10		11	10
Liabilities assumed in acquisition of Level 3(1)	—		—	58
Liabilities settled	(8)		(14)	(14)
Change in estimate	—		10	21
Balance at end of year	$	199	197	190

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(1)The liabilities assumed during 2018 relate to purchase price adjustments during the year.

The 2019 and 2018 change in estimates are offset against gross property, plant and equipment.