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 as 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.

	Year ended December 31,
	2019		2018		2017
Client Markets:
Energy, environment, and infrastructure	$	665,185	$	564,736	$	487,001
Health, education, and social programs		552,600		535,578		518,675
Safety and security		120,078		111,660		102,645
Consumer and financial		140,662		125,999		120,841
Total	$	1,478,525	$	1,337,973	$	1,229,162

	Year ended December 31,
	2019		2018		2017
Client Type:
U.S. federal government	$	561,022	$	546,050	$	550,794
U.S. state and local government		280,357		183,900		127,797
International government		122,307		122,186		91,318
Total Government		963,686		852,136		769,909
Commercial		514,839		485,837		459,253
Total	$	1,478,525	$	1,337,973	$	1,229,162

	Year ended December 31,
	2019		2018		2017
Contract Mix:
Time-and-materials	$	703,467	$	581,446	$	529,606
Fixed-price		562,985		526,751		480,584
Cost-based		212,073		229,776		218,972
Total	$	1,478,525	$	1,337,973	$	1,229,162

Contract assets consist primarily of unbilled amounts resulting from long-term contracts when revenue recognized exceeds the amount billed 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 $11.7 million increase in the Company’s net contract assets (liabilities) is due to the timing of work performed in relation to billing schedule timing for fixed price programs which resulted in the change in contract liabilities, particularly in our international operations. 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. The increase in contract liabilities is primarily due to advanced billing for costs in 2019. There were no material changes to contract balances due to impairments or business combinations during the period.

	December 31, 2019			December 31, 2018			Change
Contract assets	$	142,337		$	126,688		$	15,649
Contract liabilities		(37,413	)		(33,494	)		(3,919	)
Net contract assets (liabilities)	$	104,924		$	93,194		$	11,730

The Company had $1.5 billion in unfulfilled performance obligations as of December 31, 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.