Contract assets and liabilities are reflected in the Company’s condensed consolidated balance sheets within the accounts reflected below.

Accounts receivable represent amounts due from the Company’s customers who are concentrated primarily in the pharmaceutical, biotechnology, and medical device industries. Unbilled represents revenue recognized to date that has not been billed or is not yet contractually billable to the customer. In general, amounts become billable upon the achievement of negotiated contractual events, in accordance with predetermined payment schedules or when a reimbursable expense has been incurred. Amounts classified to unbilled are those billable to customers within one year from the respective balance sheet date.

Accounts receivable and unbilled, net consisted of the following (in thousands):

	As of
	March 31,			December 31,
	2019			2018
Accounts receivable	$	107,241		$	85,120
Unbilled receivables		28,252			49,361
Less: allowance for doubtful accounts		(5,869	)		(1,032	)
Total accounts receivable and unbilled, net	$	129,624		$	133,449

Unbilled receivables decreased from $49.4 million at December 31, 2018 to $28.3 million at March 31, 2019. The decrease is primarily driven by increased customer invoicing.

Advanced billings represents cash received from customers, or billed amounts per an agreed upon payment schedule, in advance of services being performed or revenue being recognized.

	As of
	March 31,		December 31,
	2019		2018
Advanced billings	$	150,243	$	147,935

Advanced billings increased from $147.9 million at December 31, 2018 to $150.2 million at March 31, 2019. The increase is primarily driven by billing and/or collection activity in the three months ended March 31, 2019 in advance of revenue being earned for services performed.

On March 31, 2019, we had approximately $1,156.2 million of performance obligations remaining to be performed for active projects.