Certain Balance Sheet Components

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



Accounts receivable, net	June 30, 2018		December 31, 2017
	(in thousands)
Trade receivables (net of allowance of $4,552 and $3,957)	$	412,848	$	369,047
Unbilled receivables	30,546		28,982
Total accounts receivable, net	$	443,394	$	398,029



Allowance for doubtful accounts activity	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
	(in thousands)
Beginning balance	$	4,774		$	3,424		$	3,957		$	3,320
Provision for doubtful accounts, net	334			441			1,254			744
Accounts written-off	(247		)	(475		)	(505		)	(805		)
Effect of change in exchange rates	(309		)	112			(154		)	243
Ending balance	$	4,552		$	3,502		$	4,552		$	3,502



Property, plant, and equipment, net	June 30, 2018			December 31, 2017
	(in thousands)
Machinery and equipment	$	310,855		$	310,753
Computers and software	110,758			104,384
Buildings, furniture, and improvements	145,893			135,566
Land	15,457			18,433
Construction in progress, including purchased equipment	45,370			39,946
Total cost	628,333			609,082
Accumulated depreciation	(404,898		)	(408,314		)
Property, plant, and equipment, net	$	223,435		$	200,768