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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)
Land
N/A
 
$
883

 
563

Fiber, conduit and other outside plant(1)
15-45 years
 
22,798

 
16,996

Central office and other network electronics(2)
3-10 years
 
18,538

 
13,768

Support assets(3)
3-30 years
 
7,586

 
6,623

Construction in progress(4)
N/A
 
1,399

 
1,244

Gross property, plant and equipment
 
 
51,204

 
39,194

Accumulated depreciation
 
 
(24,352
)
 
(22,155
)
Net property, plant and equipment
 
 
$
26,852

 
17,039

_______________________________________________________________________________
(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 $2.710 billion, $2.691 billion and $2.836 billion for the years ended December 31, 2017, 2016 and 2015, respectively.
Asset Retirement Obligations
At December 31, 2017, 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 preliminary estimates of fair values of Level 3's asset retirement obligations. Our preliminary fair value estimates were determined using discounted cash flow method.
The following table provides asset retirement obligation activity:
 
Years Ended December 31,
 
2017
 
2016
 
2015
 
(Dollars in millions)
Balance at beginning of year
$
95

 
91

 
107

Accretion expense
6

 
6

 
7

Liabilities assumed in acquisition of Level 3
45

 
—

 
—

Liabilities settled
(3
)
 
(2
)
 
(2
)
Liabilities transferred to Cyxtera
(20
)
 
—

 
—

Change in estimate
(8
)
 
—

 
(21
)
Balance at end of year
$
115

 
95

 
91


We revised our estimates for the cost of removal of network equipment, asbestos remediation, and other obligations by $8 million and $21 million, for the years ended December 31, 2017 and 2015, respectively. These revisions resulted in a reduction of the asset retirement obligation and offsetting reduction to gross property, plant and equipment, and revisions to assets specifically identified are recorded as a reduction to accretion expense. We did not revise our estimates for the cost of removal of network equipment, asbestos remediation, and other obligations during 2016.