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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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