The Company’s contracts contain both fixed price and cost reimbursable components. Contract types are based on the component that represents the majority of the contract. The following table presents revenue disaggregated by contract type (in thousands):

	December 31, 2018		December 31, 2019
Cost plus	$	1,473,815	$	1,705,832
Time-and-Materials		961,759		1,074,037
Fixed price		1,124,934		1,174,943
Total	$	3,560,508	$	3,954,812

Refer to “Note 21—Segment Information” for the Company’s revenues by business lines.

Contract assets and contract liabilities balances at December 31, 2018 and December 31, 2019 were as follows (in thousands):

	December 31, 2018		December 31, 2019		$ change		% change
Contract assets	$	515,319	$	575,089	$	59,770		11.60	%
Contract liabilities		208,576		230,681		22,105		10.60	%
Net contract assets (liabilities)(1)	$	306,743	$	344,408	$	37,665		12.28	%

(1)

Total contract retentions included in net contract assets (liabilities) were $89.6 million as of December 31, 2018. Total contract retentions included in net contract assets (liabilities) were $85.5 million as of December 31, 2019, of which $41.7 million are not expected to be paid in fiscal 2020. Contract assets at December 31, 2018 and December 31, 2019 include approximately $47.1 million and $73.0 million, respectively, related to unapproved change orders, claims, and requests for equitable adjustment. For the years ended December 31, 2018 and December 31, 2019, no material losses were recognized related to the collectability of claims, unapproved change orders, and requests for equitable adjustment.

During the years ended December 31, 2018 and December 31, 2019, the Company recognized revenue of approximately $168.6 million and $129.9 million, respectively that was included in the corresponding contract liability balance at December 29, 2017 and December 31, 2018, respectively. The change in contract assets and contract liabilities was the result of normal business activity and not significantly impacted by other factors, except as follows:

	December 31, 2018		December 31, 2019
Acquired contract assets	$	35,229	$	9,747
Acquired contract liabilities		3,529		1,300
Change in the estimate of variable consideration		—		12,166
Reversal of provision for contract losses(1)	$	133,180	$	—

There was no significant impairment of contract assets recognized during the years ended December 31, 2018 and December 31, 2019.

Revisions in estimates, such as changes in estimated claims or incentives, related to performance obligations partially satisfied in previous periods that individually had an impact of $5 million or more on revenue resulted in an increase in revenue of $12.1 million for the year ended December 31, 2019, and no impact for the year ended December 31, 2018.

Accounts receivable, net consisted of the following as of December 31, 2018 and December 31, 2019 (in thousands):

	2018			2019
Billed	$	538,808		$	494,366
Unbilled		135,180			218,959
Total accounts receivable, gross		673,988			713,325
Allowance for doubtful accounts		(50,702	)		(41,833	)
Total accounts receivable, net	$	623,286		$	671,492

Billed accounts receivable represents amounts billed to clients that have not been collected. Unbilled accounts receivable represents amounts where the Company has a present contractual right to bill but an invoice has not been issued to the customer at the period-end date.

The allowance for doubtful accounts was determined based on consideration of trends in actual and forecasted credit quality of clients, including delinquency and payment history, type of client, such as a government agency or commercial sector client, and general economic conditions and particular industry conditions that may affect a client’s ability to pay.

Transaction Price Allocated to the Remaining Unsatisfied Performance Obligations

The Company’s remaining unsatisfied performance obligations (“RUPO”) as of December 31, 2019 represent a measure of the total dollar value of work to be performed on contracts awarded and in progress. The Company had $5.0 billion in RUPO as of December 31, 2019.

RUPO will increase with awards of new contracts and decrease as the Company performs work and recognizes revenue on existing contracts. Projects are included within RUPO at such time the project is awarded and agreement on contract terms has been reached. The difference between RUPO and backlog relates to unexercised option years that are included within backlog and the value of Indefinite Delivery/Indefinite Quantity (“IDIQ”) contracts included in backlog for which task orders have not been issued.

RUPO is comprised of: (a) original transaction price, (b) change orders for which written confirmations from our customers have been received, (c) pending change orders for which the Company expects to receive confirmations in the ordinary course of business, and (d) claim amounts that the Company has made against customers for which it has determined that it has a legal basis under existing contractual arrangements and a significant reversal of revenue is not probable, less revenue recognized to-date.

The Company expects to satisfy its RUPO as of December 31, 2019 over the following periods (in thousands):

	Within One Year		Within One to Two Years		Thereafter
Federal Solutions	$	1,207,900	$	451,278	$	176,103
Critical Infrastructure		1,634,625		632,273		859,966
Total RUPO	$	2,842,525	$	1,083,551	$	1,036,069