Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Disaggregation of Revenue
Consulting services revenues	$ 121,762	$ 115,686	$ 370,951	$ 332,108
United States
Disaggregation of Revenue
Consulting services revenues	96,117	92,217	296,273	263,221
United Kingdom
Disaggregation of Revenue
Consulting services revenues	20,358	17,116	57,355	53,027
Other
Disaggregation of Revenue
Consulting services revenues	5,287	6,353	17,323	15,860
Fixed Price
Disaggregation of Revenue
Consulting services revenues	26,197	30,057	85,968	76,622
Time-and-materials
Disaggregation of Revenue
Consulting services revenues	$ 95,565	$ 85,629	$ 284,983	$ 255,486