Revenue - Schedules of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 1,849,441	$ 1,663,112	$ 5,494,194	$ 4,923,957
Total revenue (as a percent)	100.00%	100.00%	100.00%	100.00%
Prime Contractor
Disaggregation of Revenue [Line Items]
Revenue	$ 1,714,705	$ 1,534,912	$ 5,064,657	$ 4,524,247
Total revenue (as a percent)	93.00%	92.00%	92.00%	92.00%
Sub-contractor
Disaggregation of Revenue [Line Items]
Revenue	$ 134,736	$ 128,200	$ 429,537	$ 399,710
Total revenue (as a percent)	7.00%	8.00%	8.00%	8.00%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 1,773,317	$ 1,590,044	$ 5,304,855	$ 4,739,971
Total revenue (as a percent)	96.00%	96.00%	97.00%	96.00%
Defense Clients | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 904,389	$ 767,340	$ 2,608,595	$ 2,280,892
Total revenue (as a percent)	49.00%	46.00%	48.00%	46.00%
Intelligence Clients | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 383,422	$ 378,043	$ 1,207,709	$ 1,161,256
Total revenue (as a percent)	21.00%	23.00%	22.00%	24.00%
Civil Clients | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 485,506	$ 444,661	$ 1,488,551	$ 1,297,823
Total revenue (as a percent)	26.00%	27.00%	27.00%	26.00%
Global Commercial Clients
Disaggregation of Revenue [Line Items]
Revenue	$ 76,124	$ 73,068	$ 189,339	$ 183,986
Total revenue (as a percent)	4.00%	4.00%	3.00%	4.00%
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenue	$ 1,052,313	$ 901,660	$ 3,104,961	$ 2,612,938
Total revenue (as a percent)	57.00%	54.00%	57.00%	53.00%
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	$ 428,081	$ 376,368	$ 1,272,281	$ 1,172,461
Total revenue (as a percent)	23.00%	23.00%	23.00%	24.00%
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	$ 369,047	$ 385,084	$ 1,116,952	$ 1,138,558
Total revenue (as a percent)	20.00%	23.00%	20.00%	23.00%