Acquisition of Citizen Engagement Centers Business - Contribution of the acquired business (Details) - General Dynamics Information Technology's Citizen Engagement Centers - USD ($) $ in Thousands  | 
3 Months Ended | 9 Months Ended | 
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Jun. 30, 2019  | 
Jun. 30, 2019  | 
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| Business Acquisition [Line Items] | ||
| Revenue contributed from acquired business | $ 163,381 | $ 440,647 | 
| Gross profit contributed from acquired business | $ 31,943 | $ 83,563 | 
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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. No definition available. 
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- Definition This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Details 
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