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):

	Three Months Ended				Nine Months Ended
	September 28, 2018		September 30, 2019		September 28, 2018		September 30, 2019
Fixed-Price	$	300,225	$	288,316	$	833,224	$	850,658
Time-and-Materials		254,435		285,586		732,555		810,656
Cost-Plus		421,497		449,375		1,065,789		1,256,110
Total	$	976,157	$	1,023,277	$	2,631,568	$	2,917,424

See “Note 19 – Segments Information” for the Company’s revenues by business lines.

Contract assets and contract liabilities balances at December 31, 2018 and September 30, 2019 were as follows (in thousands):

	December 31, 2018		September 30, 2019		$ change		% change
Contract assets	$	515,319	$	583,670	$	68,351		13.3	%
Contract liabilities		208,576		231,032		22,456		10.8	%
Net contract assets (liabilities) (1)	$	306,743	$	352,638	$	45,895		15.0	%

During the three months ended September 28, 2018 and September 30, 2019, the Company recognized revenue of $0.8 million and $2.1 million, respectively, and $100.4 million and $115.1 million during the nine months ended September 28, 2018 and September 30, 2019, respectively, that was included in the corresponding contract liability balance at December 30, 2017 and December 31, 2018, respectively. The changes in contract assets and contract liabilities were the result of normal business activity and not significantly impacted by other factors, except as follows:

	December 31, 2018		September 30, 2019
Acquired contract assets	$	35,229	$	9,747
Acquired contract liabilities		3,529		1,300
Change in the estimate of variable consideration		-		14,506
Reversal of provision for contract losses (1)	$	133,180	$	-

There was no significant impairment of contract assets recognized during the three and nine months ended September 28, 2018 and September 30, 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 $9.1 million for the three and nine months ended September 30, 2019, and no amounts for the three and nine months ended September 28, 2018.

Accounts receivable, net consisted of the following as of December 31, 2018 and September 30, 2019 (in thousands):

	December 31, 2018			September 30, 2019
Billed	$	538,808		$	516,099
Unbilled		135,180			200,477
Total accounts receivable, gross		673,988			716,576
Allowance for doubtful accounts		(50,702	)		(42,902	)
Total accounts receivable, net	$	623,286		$	673,674

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 September 30, 2019 represent a measure of the total dollar value of work to be performed on contracts awarded and in progress. The Company had $5.1 billion in RUPO as of September 30, 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 delivery 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 September 30, 2019 over the following periods (in thousands):

Period RUPO Will Be Satisfied	Within One Year		Within One to Two Years		Thereafter
Federal Solutions	$	1,198,634	$	436,620	$	208,734
Critical Infrastructure		1,636,525		960,469		643,170
Total	$	2,835,159	$	1,397,089	$	851,904