Certain Balance Sheet Components

A summary of accounts receivable from contracts with customers is as follows:



Accounts receivable, net
In thousands	March 31, 2019		December 31, 2018
Trade receivables (net of allowance of $4,046 and $6,331)	$	438,692	$	416,503
Unbilled receivables	34,385		20,658
Total accounts receivable, net	$	473,077	$	437,161



Allowance for doubtful accounts activity	Three Months Ended March 31,
In thousands	2019			2018
Beginning balance	$	6,331		$	3,957
Provision for doubtful accounts, net	(2,103		)	920
Accounts written-off	(192		)	(258		)
Effect of change in exchange rates	10			155
Ending balance	$	4,046		$	4,774



Inventories
In thousands	March 31, 2019		December 31, 2018
Materials	$	132,050	$	133,398
Work in process	9,106		9,744
Finished goods	79,941		77,532
Total inventories	$	221,097	$	220,674



Property, plant, and equipment, net
In thousands	March 31, 2019			December 31, 2018
Machinery and equipment	$	318,695		$	315,974
Computers and software	107,703			104,290
Buildings, furniture, and improvements	147,320			146,071
Land	15,063			14,980
Construction in progress, including purchased equipment	45,237			49,682
Total cost	634,018			630,997
Accumulated depreciation	(409,080		)	(404,446		)
Property, plant, and equipment, net	$	224,938		$	226,551



Depreciation expense	Three Months Ended March 31,
In thousands	2019		2018
Depreciation expense	$	12,384	$	13,332

Subsequent to March 31, 2019, we entered into sales contracts for properties in Massy, France and Stretford, United Kingdom, which properties are classified as held-for-sale within other long-term assets. The estimated gains on sale are $1.5 million and $5.0 million, respectively. The Massy, France gain will be classified within operating expenses as a gain on sale of assets, and the Stretford, United Kingdom gain will offset restructuring expense as this property was included in a previous restructuring plan.