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.

Changes in the three and six months ended June 30, 2020 compared to the prior year period were primarily from the increase of revenue in the health, education, and social programs client market as a result of additional work performed for U.S. government clients, including the ITG acquisition, offset by decreases in revenue from the commercial clients primarily in the consumer and financial services and health, education and social programs markets and decreases in the U.S. state and local clients in the energy, environment, and infrastructure markets. Similarly, for the three and six months ended June 30, 2020 revenue from time-and-materials and cost-based contracts increased while revenue from fixed price contracts decreased compared to the prior year period.

	Three Months Ended June 30,										Six Months Ended June 30,
	2020					2019					2020					2019
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Client Markets:
Energy, environment, and infrastructure	$	151,481		43	%	$	166,520		45	%	$	303,839		43	%	$	320,035		45	%
Health, education, and social programs		148,528		42	%		134,346		37	%		291,114		41	%		256,255		36	%
Safety and security		31,440		9	%		30,030		8	%		63,025		9	%		59,581		9	%
Consumer and financial services		22,538		6	%		35,821		10	%		54,247		7	%		72,100		10	%
Total	$	353,987		100	%	$	366,717		100	%	$	712,225		100	%	$	707,971		100	%

	Three Months Ended										Six Months Ended June 30,
	June 30, 2020					June 30, 2019					2020					2019
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Client Type:
U.S. federal government	$	170,748		48	%	$	141,253		38	%	$	326,318		46	%	$	273,465		39	%
U.S. state and local government		57,982		17	%		73,101		20	%		119,288		17	%		138,928		20	%
International government		18,098		5	%		31,617		9	%		41,190		6	%		58,751		8	%
Total Government		246,828		70	%		245,971		67	%		486,796		69	%		471,144		67	%
Commercial		107,159		30	%		120,746		33	%		225,429		31	%		236,827		33	%
Total	$	353,987		100	%	$	366,717		100	%	$	712,225		100	%	$	707,971		100	%

	Three Months Ended										Six Months Ended June 30,
	June 30, 2020					June 30, 2019					2020					2019
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Contract Mix:
Time-and-materials	$	168,489		48	%	$	167,009		46	%	$	336,639		47	%	$	322,191		45	%
Fixed price		123,970		35	%		146,967		40	%		257,122		36	%		281,416		40	%
Cost-based		61,528		17	%		52,741		14	%		118,464		17	%		104,364		15	%
Total	$	353,987		100	%	$	366,717		100	%	$	712,225		100	%	$	707,971		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 June 30, 2020 increased by $15.5 million as compared to December 31, 2019. The increase in net contract assets (liabilities) is primarily due to work in the health, education and social programs, including the ITG acquisition, and energy, environmental, and infrastructure client markets, offset by consumer and financial services markets. There were no material changes to contract balances due to impairments during the period.

	June 30, 2020			December 31, 2019			$ Change		% Change
Contract assets	$	150,577		$	142,337		$	8,240		5.8	%
Contract liabilities		(30,135	)		(37,413	)		7,278		(19.5	%)
Net contract assets (liabilities)	$	120,442		$	104,924		$	15,518		14.8	%

The Company had $1.5 billion in unfulfilled performance obligations as of June 30, 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.