4. RECEIVABLES, CONTRACT ASSETS, AND DEFERRED REVENUE

Receivables



(in thousands)	December 31, 2018		December 31, 2017
Accounts receivable	$	180,872	$	222,735
Unbilled receivables	172,656		160,084
Long-term unbilled receivables	151,237		160,708
	$	504,765	$	543,527

Unbilled receivables are the amounts due from clients where the only condition to the right of payment is the passage of time. As of December 31, 2018 and 2017, the allowance for doubtful accounts was not material.

Unbilled receivables are expected to be billed in the future as follows:



(Dollars in thousands)	December 31, 2018
1 Year or less	$	172,656	54	%
1-2 Years	95,013		29	%
2-5 Years	56,224		17	%
	$	323,893	100	%

Contract assets and deferred revenue



(in thousands)	December 31, 2018		December 31, 2017
Contract assets ⁽¹⁾	$	3,711	$	914
Long-term contract assets ⁽²⁾	2,543		—
	$	6,254	$	914
Deferred revenue	$	185,145	$	165,850
Long-term deferred revenue ⁽³⁾	5,344		3,706
	$	190,489	$	169,556

⁽¹⁾Included in other current assets.

⁽²⁾ Included in other long-term assets.

⁽³⁾Included in other long-term liabilities.

Contract assets occur when revenue recognized exceeds the amount billed to the client and the right to payment is subject to conditions other than the passage of time, such as the completion of a related performance obligation. Deferred revenue consists of billings and payments received in advance of revenue recognition. Contract assets and deferred revenue are netted at the contract level for each reporting period. There were no material impairments of contract assets for 2018, 2017, or 2016.

The change in deferred revenue in 2018 was primarily due to new billings in advance of revenue recognition, partially offset by $237.5 million of revenue recognized, excluding the impact of netting contract assets and deferred revenue at the contract level, during the period that was included in deferred revenue at December 31, 2017.

Major clients

No client represented 10% or more of the Company’s total accounts receivable and unbilled receivables as of December 31, 2018 or December 31, 2017.