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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2018
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,
 
 
2018
 
2017
 
 
 
(Dollars in millions)
Land
N/A
 
$
871

 
883

Fiber, conduit and other outside plant(1)
15-45 years
 
23,936

 
22,798

Central office and other network electronics(2)
3-10 years
 
18,736

 
18,538

Support assets(3)
3-30 years
 
8,020

 
7,586

Construction in progress(4)
N/A
 
1,704

 
1,399

Gross property, plant and equipment
 
 
53,267

 
51,204

Accumulated depreciation
 
 
(26,859
)
 
(24,352
)
Net property, plant and equipment
 
 
$
26,408

 
26,852

_______________________________________________________________________________
(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 $3.3 billion, $2.7 billion and $2.7 billion for the years ended December 31, 2018, 2017 and 2016, respectively.

Asset Retirement Obligations

At December 31, 2018, 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.

As of the Level 3 acquisition date, we recorded liabilities to reflect our fair values of Level 3's asset retirement obligations. Our fair value estimates were determined using discounted cash flow method.

The following table provides asset retirement obligation activity:
 
Years Ended December 31,
 
2018
 
2017
 
2016
 
(Dollars in millions)
Balance at beginning of year
$
115

 
95

 
91

Accretion expense
10

 
6

 
6

Liabilities assumed in acquisition of Level 3(1)
58

 
45

 
—

Liabilities settled
(14
)
 
(3
)
 
(2
)
Liabilities transferred to Cyxtera
—

 
(20
)
 
—

Change in estimate
21

 
(8
)
 
—

Balance at end of year
$
190

 
115

 
95

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


During 2018, we revised our estimates for the cost of removal of network equipment, tanks, and asbestos remediation resulting in a $21 million change in estimate of our asset retirement obligation. The 2018 and 2017 change in estimates are offset to gross property, plant and equipment.