Reportable Segments - Revenue by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	$ 162,689	$ 127,974	$ 328,169	$ 245,309
INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	87,889	82,790	172,315	160,062
BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	36,641	45,184	79,752	85,247
GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	38,159	0	76,102	0
Cost-reimbursable contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	151,957	115,969	303,475	223,879
Cost-reimbursable contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	82,631	79,974	162,996	155,740
Cost-reimbursable contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	31,167	35,995	64,377	68,139
Cost-reimbursable contracts | GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	38,159	0	76,102	0
Fixed-unit price contracts
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	10,732	12,005	24,694	21,430
Fixed-unit price contracts | INF
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	5,258	2,816	9,319	4,322
Fixed-unit price contracts | BTS
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	5,474	9,189	15,375	17,108
Fixed-unit price contracts | GEO
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	$ 0	$ 0	$ 0	$ 0