Business Overview - Narrative (Details)	9 Months Ended
Jun. 30, 2021 employee location customer market business_area	Jun. 30, 2020
Concentration Risk [Line Items]
Number of market focus areas \| business_area	3
Number of market leveraging core competencies \| market	7
Number of largest customers \| customer	3
Revenue from Contract with Customer \| Customer concentration
Concentration Risk [Line Items]
Percent of total revenue	100.00%	100.00%
Revenue from Contract with Customer \| Customer concentration \| Federal Government
Concentration Risk [Line Items]
Percent of total revenue	99.00%
U.S.
Concentration Risk [Line Items]
Number of employees \| employee	2,200
Number of locations in which entity operates	30
Overseas
Concentration Risk [Line Items]
Number of locations in which entity operates	1

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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