Property, Plant and Equipment

12 Months Ended

Dec. 31, 2015

Property, Plant and Equipment [Abstract]

Net property, plant and equipment is composed of the following:



	Depreciable Lives	As of December 31,
	Depreciable Lives	2015			2014
		(Dollars in millions)
Land	n/a	$	571		575
Fiber, conduit and other outside plant ⁽¹⁾	15-45	16,166			15,151
Central office and other network electronics ⁽²⁾	3-10	14,144			13,248
Support assets ⁽³⁾	3-30	7,000			6,578
Construction in progress ⁽⁴⁾	n/a	904			1,166
Gross property, plant and equipment		38,785			36,718
Accumulated depreciation		(20,716		)	(18,285	)
Net property, plant and equipment		$	18,069		18,433

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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, 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 $2.836 billion, $2.958 billion and $2.952 billion for the years ended December 31, 2015, 2014 and 2013, respectively.

In 2014, we recorded an impairment charge of $17 million in connection with a sale-leaseback transaction involving an office building which closed in the fourth quarter of 2014. This impairment charge is included in selling, general and administrative expense in our consolidated statement of operations for the year ended December 31, 2014.

Additionally, in 2014 we sold an office building for $12 million.

Asset Retirement Obligations

At December 31, 2015, 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.

The following table provides asset retirement obligation activity:



	Years Ended December 31,
	2015			2014		2013
	(Dollars in millions)
Balance at beginning of year	$	107		106		106
Accretion expense	7			7		7
Liabilities incurred	—			6		—
Liabilities settled	(2		)	(2	)	(4	)
Change in estimate	(21		)	(10	)	(3	)
Balance at end of year	$	91		107		106

During 2015, 2014 and 2013, we revised our estimates for the cost of removal of network equipment, asbestos remediation, and other obligations by $21 million, $10 million and $3 million, 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.