Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Disaggregation of Revenue
Consulting services revenues	$ 119,262	$ 115,686	$ 110,573	$ 105,849	$ 108,763	$ 103,871	$ 105,538	$ 99,476	$ 451,370	$ 417,648	$ 370,075
United States
Disaggregation of Revenue
Consulting services revenues									357,156	329,678	295,232
United Kingdom
Disaggregation of Revenue
Consulting services revenues									72,169	65,874	53,644
Other
Disaggregation of Revenue
Consulting services revenues									22,045	22,096	21,199
Fixed Price
Disaggregation of Revenue
Consulting services revenues									107,344	95,096	93,570
Time-and-materials
Disaggregation of Revenue
Consulting services revenues									$ 344,026	$ 322,552	$ 276,505