Reportable Segments - Revenue by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	$ 165,480	$ 117,335
INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	84,426	77,273
BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	43,112	40,062
GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	37,942	0
Cost-reimbursable contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	151,518	107,911
Cost-reimbursable contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	80,365	75,767
Cost-reimbursable contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	33,211	32,144
Cost-reimbursable contracts | GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	37,942	0
Fixed-unit price contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	13,962	9,424
Fixed-unit price contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	4,061	1,506
Fixed-unit price contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	9,901	7,918
Fixed-unit price contracts | GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	$ 0	$ 0