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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2017
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,
 
 
2017
 
2016
 
 
 
(Dollars in millions)
Property, plant and equipment:
 
 
 
 
 
Land
N/A
 
$
333

 
348

Fiber, conduit and other outside plant(1)
15-45 years
 
6,639

 
5,980

Central office and other network electronics(2)
4-10 years
 
4,250

 
3,855

Support assets(3)
5-30 years
 
2,620

 
2,633

Construction in progress(4)
N/A
 
474

 
431

Gross property, plant and equipment
 
 
14,316

 
13,247

Accumulated depreciation
 
 
(6,392
)
 
(5,602
)
Net property, plant and equipment
 
 
$
7,924

 
7,645

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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, 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 $912 million, $924 million and $986 million for the years ended December 31, 2017, 2016 and 2015, respectively.