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Major Clients (Details) - Revenue - Customer Concentration Risk	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Percentage of total revenue
Period for which criteria of threshold percentage for disclosure not met in past	5 years
Maximum
Percentage of total revenue
Percentage of revenue accounted for by no other single client (as a percent)	10.00%
Agencies of U.S. Federal Government
Percentage of total revenue
Concentration (as a percent)	26.00%	23.00%	22.00%
Agencies of U.S. Federal Government | Management Services
Percentage of total revenue
Concentration (as a percent)	3.00%	2.00%	3.00%

X

- Definition

Represents the period for which criteria of threshold percentage for disclosure not met in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Threshold percentage which the entity uses for disclosure.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

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.

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