Reportable Segments - Revenue by Contract Type (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
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	$ 132,598	$ 131,032	$ 127,974	$ 117,335	$ 115,344	$ 104,185	$ 104,018	$ 94,534	$ 508,938	$ 418,081	$ 333,034
INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									331,161	254,723
BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									177,777	163,358
Cost-reimbursable contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									457,518	383,103
Cost-reimbursable contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									318,112	254,365
Cost-reimbursable contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									139,406	128,738
Fixed-unit price contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									51,420	34,978
Fixed-unit price contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									13,049	358
Fixed-unit price contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues									$ 38,371	$ 34,620