Accounts Receivable

12 Months Ended

Dec. 31, 2017

Accounts, Notes, Loans and Financing Receivable, Gross, Allowance, and Net [Abstract]

Accounts Receivable

Accounts Receivable

Net accounts receivable consisted of the following as of December 31, 2017 and 2016 (in thousands):

	December 31, 2017			December 31, 2016
Trade accounts receivable	$	319,996		$	260,976
Unbilled accounts receivable	140,410			113,765
Gross accounts receivable	460,406			374,741
Allowance for doubtful accounts	(1,043		)	(829		)
Reserve for cash-basis customers	(236		)	(5,316		)
Total accounts receivable reserves	(1,279		)	(6,145		)
Accounts receivable, net	$	459,127		$	368,596

A summary of activity in the accounts receivable reserves for the years ended December 31, 2017, 2016 and 2015, is as follows (in thousands):

	2017			2016			2015
Beginning balance	$	6,145		$	7,364		$	9,023
Charges to income from operations	5,809			49,677			37,870
Collections from cash basis customers and write-offs	(10,675		)	(50,896		)	(39,529		)
Ending balance	$	1,279		$	6,145		$	7,364

Charges to income from operations represent charges to bad debt expense for increases in the allowance for doubtful accounts and reductions to revenue for increases in reserves for cash basis customers. The decrease in the reserve activity during 2017 is primarily attributable to two customers that were removed from cash-basis revenue recognition due to a strong, consistent history of payment.