Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 725,515	$ 589,989	$ 612,445	$ 568,360	$ 624,177	$ 523,711	$ 554,208	$ 530,738	$ 2,496,309	$ 2,232,834	$ 2,124,074
Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									2,497,681	2,235,841	2,132,179
Eliminations
Disaggregation of Revenue [Line Items]
Revenues									(1,372)	(3,007)	(8,105)
Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues									1,229,635	1,235,993	1,254,195
Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues									781,374	634,205	614,852
Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues									337,598	244,063	165,519
Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues									149,074	121,580	97,613
Over time
Disaggregation of Revenue [Line Items]
Revenues									2,405,009	2,168,586	2,080,438
Point-in-time
Disaggregation of Revenue [Line Items]
Revenues									92,672	67,255	51,741
United States:
Disaggregation of Revenue [Line Items]
Revenues									2,065,929	1,825,778	1,758,601
United States: \| Government
Disaggregation of Revenue [Line Items]
Revenues									1,884,671	1,703,005	1,624,078
United States: \| Non-Government
Disaggregation of Revenue [Line Items]
Revenues									181,258	122,773	134,523
Canada:
Disaggregation of Revenue [Line Items]
Revenues									390,257	376,918	348,829
Canada: \| Government
Disaggregation of Revenue [Line Items]
Revenues									245	79	0
Canada: \| Non-Government
Disaggregation of Revenue [Line Items]
Revenues									390,012	376,839	348,829
Other:
Disaggregation of Revenue [Line Items]
Revenues									41,495	33,145	24,749
Other: \| Government
Disaggregation of Revenue [Line Items]
Revenues									10,016	1,356	0
Other: \| Non-Government
Disaggregation of Revenue [Line Items]
Revenues									31,479	31,789	24,749
Government Operations \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									2,031,337	1,808,483	1,725,097
Government Operations \| Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues									1,218,516	1,226,265	1,244,916
Government Operations \| Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues									469,138	334,076	307,652
Government Operations \| Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues									337,598	244,063	165,519
Government Operations \| Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues									6,085	4,079	7,010
Government Operations \| Over time
Disaggregation of Revenue [Line Items]
Revenues									2,012,949	1,798,388	1,724,961
Government Operations \| Point-in-time
Disaggregation of Revenue [Line Items]
Revenues									18,388	10,095	136
Government Operations \| United States: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									2,008,275	1,794,658	1,714,499
Government Operations \| United States: \| Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									1,884,671	1,703,005	1,624,078
Government Operations \| United States: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									123,604	91,653	90,421
Government Operations \| Canada: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									1,023	3,213	3,417
Government Operations \| Canada: \| Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									245	79
Government Operations \| Canada: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									778	3,134	3,417
Government Operations \| Other: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									22,039	10,612	7,181
Government Operations \| Other: \| Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									10,016	1,356
Government Operations \| Other: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									12,023	9,256	7,181
Commercial Operations \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									466,344	427,358	407,082
Commercial Operations \| Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues									11,119	9,728	9,279
Commercial Operations \| Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues									312,236	300,129	307,200
Commercial Operations \| Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues									0	0	0
Commercial Operations \| Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues									142,989	117,501	90,603
Commercial Operations \| Over time
Disaggregation of Revenue [Line Items]
Revenues									392,060	370,198	355,477
Commercial Operations \| Point-in-time
Disaggregation of Revenue [Line Items]
Revenues									74,284	57,160	51,605
Commercial Operations \| United States: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									57,654	31,120	44,102
Commercial Operations \| United States: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									57,654	31,120	44,102
Commercial Operations \| Canada: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									389,234	373,705	345,412
Commercial Operations \| Canada: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									389,234	373,705	345,412
Commercial Operations \| Other: \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									19,456	22,533	17,568
Commercial Operations \| Other: \| Non-Government \| Reportable Segments
Disaggregation of Revenue [Line Items]
Revenues									$ 19,456	$ 22,533	$ 17,568