2. Balance Sheet Details

The following tables provide details of selected balance sheet accounts (in thousands):

	June 30, 2014			September 30, 2014
Accounts receivable
Billed receivables	$	189,489		$	160,202
Unbilled receivables	1,975			2,552
Less allowance for doubtful accounts	(5,691		)	(6,905		)
Total	$	185,773		$	155,849

	June 30, 2014			September 30, 2014
Property and equipment, net
Land	$	13,651		$	13,651
Buildings and improvements	163,952			166,468
Leasehold improvements	9,744			10,058
Equipment and tooling	154,367			154,501
Furniture and fixtures	4,017			4,052
Computer equipment	17,466			17,810
Computer software	15,670			15,968
Construction in process	12,650			10,612
Total	391,517			393,120
Less: accumulated depreciation and amortization	(131,038		)	(142,927		)
Property and equipment, net	$	260,479		$	250,193

During the three months ended September 30, 2013 and 2014, depreciation expense was approximately $11.5 million and $16.9 million, respectively.