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Significant Accounting Policies - Revenue (Details) $ in Billions	12 Months Ended
	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Revenue Recognition
Subcontractor and other direct costs | $	$ 10.7	$ 9.2	$ 8.4
Cost Reimbursable Contract
Revenue Recognition
Types of contracts (number)	2
Cost Reimbursable Contract | Revenue
Revenue Recognition
Concentration (as a percent)	47.00%	48.00%	53.00%
Cost Plus Contract
Revenue Recognition
Types of contracts (number)	2
Guaranteed Maximum Price Contract | Revenue
Revenue Recognition
Concentration (as a percent)	23.00%	23.00%	15.00%
Fixed-price Contract
Revenue Recognition
Types of contracts (number)	2
Fixed-price Contract | Revenue
Revenue Recognition
Concentration (as a percent)	30.00%	29.00%	32.00%

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Other costs incurred and are directly related to services rendered by an entity during the reporting period.

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-Name Accounting Standards Codification
-Topic 225
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-Section S99
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