The Company disaggregates revenue from clients, most of which is earned over time, into categories that depict how the nature, amount and uncertainty of revenue and cash flows are affected by economic factors. Those categories are client market, client type and contract mix. Client markets provide insight into the breadth of the Company’s expertise. In classifying revenue by client market, the Company attributes revenue from a client to the market that the Company believes is the client’s primary market. The Company also classifies revenue by the type of entity for which it does business, which is an indicator of the diversity of its client base. The Company attributes revenue generated from being a subcontractor to a commercial company as government revenue when the ultimate client is a government agency or department. Disaggregation by contract mix provides insight in terms of the degree of performance risk that the Company has assumed. Fixed-price contracts are considered to provide the highest amount of performance risk as the Company is required to deliver a scope of work or level of effort for a negotiated fixed price. Time-and-materials contracts require the Company to provide skilled employees on contracts for negotiated fixed hourly rates. Since the Company is not required to deliver a scope of work, but merely skilled employees, it considers these contracts to be less risky than a fixed-price agreement. Cost-based contracts are considered to provide the lowest amount of performance risk since the Company is generally reimbursed for all contract costs incurred in performance of contract deliverables with only the amount of incentive or award fees (if applicable) dependent on the achievement of negotiated performance requirements.

Increases in revenue in the three months ended March 31, 2020 compared to the prior year period were primarily from the health, education, and social programs client market as a result of additional work performed for government clients. Revenue from time-and-materials contracts for the three months ended March 31, 2020 increased as compared to the three months ended March 31, 2019.

	Three Months Ended March 31,
	2020					2019
	Dollars		Percent			Dollars		Percent
Client Markets:
Energy, environment, and infrastructure	$	151,362		42	%	$	153,403		45	%
Health, education, and social programs		143,423		40	%		121,423		35	%
Safety and security		31,585		9	%		29,551		9	%
Consumer and financial services		31,868		9	%		36,877		11	%
Total	$	358,238		100	%	$	341,254		100	%

	Three Months Ended
	March 31, 2020					March 31, 2019
	Dollars		Percent			Dollars		Percent
Client Type:
U.S. federal government	$	155,375		44	%	$	132,092		39	%
U.S. state and local government		61,330		17	%		65,950		19	%
International government		23,092		6	%		27,135		8	%
Total Government		239,797		67	%		225,177		66	%
Commercial		118,441		33	%		116,077		34	%
Total	$	358,238		100	%	$	341,254		100	%

	Three Months Ended
	March 31, 2020					March 31, 2019
	Dollars		Percent			Dollars		Percent
Contract Mix:
Time-and-materials	$	168,213		47	%	$	155,182		45	%
Fixed price		133,128		37	%		134,448		40	%
Cost-based		56,896		16	%		51,624		15	%
Total	$	358,238		100	%	$	341,254		100	%

Contract assets consist primarily of unbilled amounts resulting from long-term contracts when revenue recognized exceeds the amount billed often due to billing schedule timing. Contract liabilities result from advance payments received on a contract or from billings in excess of revenue recognized on long-term contracts due to billing schedule timing. The net contract assets (liabilities) as of March 31, 2020 increased by $17.3 million as compared to December 31, 2019. The increase in contract assets is primarily due to work in the health, education and social programs and energy, environmental, and infrastructure client markets for the U.S. federal government, most of which has been performed on time-and-materials contracts. There were no material changes to contract balances due to impairments or business combinations during the period.

	March 31, 2020			December 31, 2019			$ Change		% Change
Contract assets	$	151,558		$	142,337		$	9,221		6.5	%
Contract liabilities		(29,318	)		(37,413	)		8,095		(21.6	%)
Net contract assets (liabilities)	$	122,240		$	104,924		$	17,316		16.5	%

The Company had $1.5 billion in unfulfilled performance obligations as of March 31, 2020, which primarily entail the future delivery of services for which revenue will be recognized over time. The obligations relate to continued or additional services required on contracts and were generally valued using an estimated cost-plus margin approach, with variable consideration being estimated at the most likely amount. The Company expects to satisfy these performance obligations, on average, in one to two years.