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Property, Plant and Equipment
12 Months Ended
Dec. 31, 2021
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment Property, Plant and Equipment
Net property, plant and equipment is composed of the following:
 Depreciable
Lives
As of December 31,
 20212020
  (Dollars in millions)
LandN/A$751 848 
Fiber, conduit and other outside plant(1)
15-45 years
15,366 26,522 
Central office and other network electronics(2)
3-10 years
15,394 20,692 
Support assets(3)
3-30 years
7,181 8,261 
Construction in progress(4)
N/A1,474 1,611 
Gross property, plant and equipment 40,166 57,934 
Accumulated depreciation (19,271)(31,596)
Net property, plant and equipment $20,895 26,338 
_______________________________________________________________________________
(1)Fiber, conduit and other outside plant consists of fiber and metallic cable, conduit, poles and other supporting structures. Fiber, conduit and other outside plant decreased as of December 31, 2021 compared to December 31, 2020 due to the retirement of a portion of our copper-based infrastructure being replaced with our Quantum Fiber infrastructure.
(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.

At December 31, 2021, we classified $5.1 billion of certain property, plant and equipment, net as held for sale and discontinued recording depreciation on these disposal groups. See Note 2—Planned Divestiture of the Latin American and ILEC Businesses for more information.

We recorded depreciation expense of $2.7 billion, $3.0 billion and $3.1 billion for the years ended December 31, 2021, 2020 and 2019, respectively.

Asset Retirement Obligations

As of December 31, 2021 and 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,
 202120202019
 (Dollars in millions)
Balance at beginning of year$199 197 190 
Accretion expense10 10 11 
Liabilities settled(13)(8)(14)
Change in estimate(2)— 10 
Reclassified as held for sale(1)
(12)— — 
Balance at end of year$182 199 197 
_______________________________________________________________________________
(1)Represents the amounts reclassified as held for sale related to our planned divestitures. See Note 2—Planned Divestiture of the Latin American and ILEC Businesses.

The 2019 and 2021 changes in estimates referred to in the table above were offset against gross property, plant and equipment.