Revenue Recognition - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Disaggregation of Revenue
Consulting services revenues	$ 99,476	$ 88,171
United States
Disaggregation of Revenue
Consulting services revenues	76,860	72,628
United Kingdom
Disaggregation of Revenue
Consulting services revenues	17,505	10,542
Countries Other Than United States, United Kingdom
Disaggregation of Revenue
Consulting services revenues	5,111	5,001
Fixed Price
Disaggregation of Revenue
Consulting services revenues	20,714	18,774
Time-and-materials
Disaggregation of Revenue
Consulting services revenues	$ 78,762	$ 69,397