Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenue	$ 1,468,711	$ 1,395,469	$ 2,928,217	$ 2,758,861
Expertise
Disaggregation Of Revenue [Line Items]
Revenue	732,276	742,034	1,472,959	1,464,387
Technology
Disaggregation Of Revenue [Line Items]
Revenue	736,435	653,435	1,455,258	1,294,474
Prime contractor
Disaggregation Of Revenue [Line Items]
Revenue	1,327,025	1,266,655	2,653,863	2,501,760
Subcontractor
Disaggregation Of Revenue [Line Items]
Revenue	141,686	128,814	274,354	257,101
Department of Defense
Disaggregation Of Revenue [Line Items]
Revenue	1,012,875	990,381	2,017,070	1,928,021
Federal civilian agencies
Disaggregation Of Revenue [Line Items]
Revenue	390,034	342,029	780,213	706,022
Commercial and other
Disaggregation Of Revenue [Line Items]
Revenue	65,802	63,059	130,934	124,818
Cost-plus-fee
Disaggregation Of Revenue [Line Items]
Revenue	843,584	818,477	1,667,193	1,566,191
Fixed-price
Disaggregation Of Revenue [Line Items]
Revenue	440,821	388,867	874,635	806,843
Time and materials
Disaggregation Of Revenue [Line Items]
Revenue	184,306	188,125	386,389	385,827
Domestic
Disaggregation Of Revenue [Line Items]
Revenue	1,427,060	1,353,793	2,845,247	2,678,566
Domestic | Expertise
Disaggregation Of Revenue [Line Items]
Revenue	715,812	726,476	1,439,009	1,435,501
Domestic | Technology
Disaggregation Of Revenue [Line Items]
Revenue	711,248	627,317	1,406,238	1,243,065
Domestic | Prime contractor
Disaggregation Of Revenue [Line Items]
Revenue	1,288,533	1,227,317	2,577,238	2,424,951
Domestic | Subcontractor
Disaggregation Of Revenue [Line Items]
Revenue	138,527	126,476	268,009	253,615
Domestic | Department of Defense
Disaggregation Of Revenue [Line Items]
Revenue	1,012,875	990,381	2,017,070	1,928,021
Domestic | Federal civilian agencies
Disaggregation Of Revenue [Line Items]
Revenue	390,034	342,029	780,213	706,022
Domestic | Commercial and other
Disaggregation Of Revenue [Line Items]
Revenue	24,151	21,383	47,964	44,523
Domestic | Cost-plus-fee
Disaggregation Of Revenue [Line Items]
Revenue	843,584	818,477	1,667,193	1,566,191
Domestic | Fixed-price
Disaggregation Of Revenue [Line Items]
Revenue	411,114	361,016	820,698	752,552
Domestic | Time and materials
Disaggregation Of Revenue [Line Items]
Revenue	172,362	174,300	357,356	359,823
International
Disaggregation Of Revenue [Line Items]
Revenue	41,651	41,676	82,970	80,295
International | Expertise
Disaggregation Of Revenue [Line Items]
Revenue	16,464	15,558	33,950	28,886
International | Technology
Disaggregation Of Revenue [Line Items]
Revenue	25,187	26,118	49,020	51,409
International | Prime contractor
Disaggregation Of Revenue [Line Items]
Revenue	38,492	39,338	76,625	76,809
International | Subcontractor
Disaggregation Of Revenue [Line Items]
Revenue	3,159	2,338	6,345	3,486
International | Commercial and other
Disaggregation Of Revenue [Line Items]
Revenue	41,651	41,676	82,970	80,295
International | Fixed-price
Disaggregation Of Revenue [Line Items]
Revenue	29,707	27,851	53,937	54,291
International | Time and materials
Disaggregation Of Revenue [Line Items]
Revenue	$ 11,944	$ 13,825	$ 29,033	$ 26,004