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 market revenue information provides 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. Finally, 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 from energy, environment, and infrastructure client market, and U.S. state and local governments in the three and nine months ended September 30, 2019 compared to the prior year period were primarily due to work performed on disaster recovery and relief efforts, which also caused an increase in revenue generated through time-and-materials contracts. Consumer and financial services client market revenue increased primarily due to acquisitions during the prior year.

	Three Months Ended September 30,										Nine Months Ended September 30,
	2019					2018					2019					2018
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Client Markets:
Energy, environment, and infrastructure	$	172,217		46	%	$	147,914		44	%	$	492,509		46	%	$	404,870		42	%
Health, education, and social programs		138,699		37	%		126,424		38	%		394,288		36	%		381,063		40	%
Safety and security		29,682		8	%		29,478		9	%		88,913		8	%		82,444		8	%
Consumer and financial services		33,320		9	%		29,152		9	%		106,179		10	%		91,686		10	%
Total	$	373,918		100	%	$	332,968		100	%	$	1,081,889		100	%	$	960,063		100	%

	Three Months Ended										Nine Months Ended September 30,
	September 30, 2019					September 30, 2018					2019					2018
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Client Type:
U.S. federal government	$	148,181		40	%	$	140,276		42	%	$	421,287		39	%	$	413,599		43	%
U.S. state and local government		71,540		19	%		55,525		17	%		210,324		19	%		121,114		13	%
International government		28,020		7	%		26,559		8	%		86,885		8	%		89,950		9	%
Total Government		247,741		66	%		222,360		67	%		718,496		66	%		624,663		65	%
Commercial		126,177		34	%		110,608		33	%		363,393		34	%		335,400		35	%
Total	$	373,918		100	%	$	332,968		100	%	$	1,081,889		100	%	$	960,063		100	%

	Three Months Ended										Nine Months Ended September 30,
	September 30, 2019					September 30, 2018					2019					2018
	Dollars		Percent			Dollars		Percent			Dollars		Percent			Dollars		Percent
Contract Mix:
Time-and-materials	$	184,330		49	%	$	146,525		44	%	$	507,926		47	%	$	398,815		42	%
Fixed price		135,274		36	%		126,126		38	%		415,258		38	%		384,751		40	%
Cost-based		54,314		15	%		60,317		18	%		158,705		15	%		176,497		18	%
Total	$	373,918		100	%	$	332,968		100	%	$	1,081,889		100	%	$	960,063		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 contact assets (liabilities) increased $27.8 million. The increase in contract assets is primarily due to hurricane relief and rebuild work for U.S. state and local governments, which is considered part of the energy, environment and infrastructure client market, and most of which has been performed on time-and-materials agreements. There were no material changes to contract balances due to impairments or business combinations during the period.

	September 30, 2019			December 31, 2018			$ Change		% Change
Contract assets	$	153,055		$	126,688		$	26,367		20.8	%
Contract liabilities		(32,086	)		(33,494	)		1,408		(4.2	%)
Net contract assets (liabilities)	$	120,969		$	93,194		$	27,775		29.8	%

The Company had $1.6 billion in unfulfilled performance obligations as of September 30, 2019, 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.