Property, Plant and Equipment

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



		Successor
	Depreciable Lives	December 31, 2018			December 31, 2017
		(Dollars in millions)
Land	N/A	$	339		348
Fiber conduit and other outside plant⁽¹⁾	15-45 years	5,262			4,750
Central office and other network electronics⁽²⁾	7-10 years	1,986			2,134
Support assets⁽³⁾	3-30 years	2,327			2,019
Construction-in-progress⁽⁴⁾	N/A	560			304
Gross property, plant and equipment		10,474			9,555
Accumulated depreciation⁽⁵⁾		(1,021		)	(143	)
Net property, plant and equipment		$	9,453		9,412

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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 construction and property of the aforementioned categories that has not been placed in service as it is still under construction.
(5)	CenturyLink's acquisition of us caused our assets and liabilities to be recognized at fair value and resulted in accumulated depreciation being reset as of the date of acquisition.

Depreciation expense was $906 million and $143 million for the successor year ended December 31, 2018 and period ended December 31, 2017, $850 million and $948 million for the predecessor period ended October 31, 2017 and for the predecessor year ended December 31, 2016.

Asset Retirement Obligations

At the successor dates of December 31, 2018 and 2017, our asset retirement obligations consisted of restoration requirements for leased facilities. At the predecessor date of December 31, 2016, our asset retirement obligations balance was primarily related to estimated future costs to remove certain of our network infrastructure at the expiration of the underlying right-of-way ("ROW") term and restoration requirements for leased facilities and estimated future costs of properly disposing of asbestos and other hazardous materials upon remodeling or demolishing buildings. We recognize our estimate of the fair value of our asset retirement obligations in the period incurred in other long-term liabilities. The fair value of the asset retirement obligation is also capitalized as property, plant and equipment and then depreciated over the estimated remaining useful life of the associated asset.

The following table provides asset retirement obligation activity:



	Successor			Successor		Predecessor
	Year Ended December 31, 2018			Period Ended December 31, 2017		Period Ended October 31, 2017		Year Ended December 31, 2016
	(Dollars in millions)
Balance at beginning of period	$	45		45		89		90
Accretion expense	5			1		12		10
Purchase price adjustments ⁽¹⁾	58			—		—		—
Liabilities settled	(13		)	(1	)	(7	)	(9	)
Revision in estimated cash flows	10			—		—		—
Effect of foreign currency rate change	—			—		—		(2	)
Balance at end of period	$	105		45		94		89

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(1)	These liabilities relate to purchase price adjustments that occurred during 2018 from CenturyLink's acquisition of us.