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 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 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,
	2018		2017		2016
Client Markets:
Energy, environment, and infrastructure	$	565,125	$	487,001	$	457,992
Health, education, and social programs		535,314		518,675		508,903
Safety and security		111,072		102,645		98,358
Consumer and financial		126,462		120,841		119,844
Total	$	1,337,973	$	1,229,162	$	1,185,097

	Year ended December 31,
	2018		2017		2016
Client Type:
U.S. federal government	$	543,918	$	550,794	$	563,498
U.S. state and local government		185,130		127,797		132,287
International government		122,293		91,318		75,636
Total Government		851,341		769,909		771,421
Commercial		486,632		459,253		413,676
Total	$	1,337,973	$	1,229,162	$	1,185,097

	Year ended December 31,
	2018		2017		2016
Contract Mix:
Time-and-materials	$	581,965	$	529,606	$	511,747
Fixed-price		526,728		480,584		456,065
Cost-based		229,280		218,972		217,285
Total	$	1,337,973	$	1,229,162	$	1,185,097

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 $8.6 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 a reduction 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 decrease in contract liabilities is primarily due to advanced billing for costs in 2017 that has been recognized as revenue in 2018. There were no material changes to contract balances due to impairments or business combinations during the period.

The Company had $1.4 billion in unfulfilled performance obligations as of December 31, 2018, 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 year.